In one-handed solitaire, after discarding cards the rules state "If there are previously drawn cards in your hand, rearrange the hand so that four cards are visible."
What does this mean?
In one-handed solitaire, your "hand" is all the face-up cards on the top of the deck. You are always supposed to have the top four of these cards visible, which can be achieved by fanning out the top four cards.
So this rule means that "if the number of visible cards is less than four, and the total number of face-up cards on the top of the deck is at least four, then fan out the top four cards so that you can see them all".
Your question has absolutely no context, so it is difficult to answer in a way that is specific to what you need.
We need the e full context of the rules to explain this. If you have trouble with terminology, see the original text of this answer, below. Emphasis mine.
Taking a standard, shuffled 52-card deck of playing cards (without Jokers), hold the deck face down in your hand. Draw from the back of the deck four cards and place them on top fanned out so that the suit and number can be seen.
As the above rule indicates, you draw cards at a certain point in this game. You always draw four cards. There is no maximum limit of cards you may have in your hand written in the quoted rules, so you may have leftover cards from a previous action. Adding four new cards to your hand can result in having more than four cards in your hand. For example, if you had one card in your hand, then drew four cards, you would have five cards in your hand.
If the first and fourth card are the same suit, discard the two middle cards, placing them on your lap if seated or in a pocket or elsewhere if standing. If there are previously drawn cards in your hand, rearrange the hand so that four cards are visible. If there are not enough cards to do this, draw from the back so that four cards are visible.
After performing prior actions, you may still have more than four cards in your hand. If you do do, you need to adjust your hand so that you can only see the faces of four cards. You shouod effectively be playing a four card hand at this point.
The linked rules don't specify about how to rearrange the cards, or what happens with the cards you can no longer see. As such, based only on those rules, you ha e the option of discarding the no-longer-accessible cards from the gake. However, this seems highly punative and very unlikey that you could win if you took this approach.
The other option, which I suspect is the intended option, and is definitely easier, is if you flip some cards face down so that you can't see them any more, or otherwise set them to the side. So, if you had 5 cards, flip 1 face down. That card wou k d effectively be out of play, amd could only come back if you have more than 4 cards on the table in the future, prompting you to rearrange your cards again (unless there's another rule that lets you rearrange under otber conditions).
---- Original answer ---- That said, let's break things down...
if there are previously drawn cards in your hand
Your 'hand' typically refers to the set of cards you are playing. You typically hold these cards in the body part also known as your hand (that which is attached to your arm, at the opposite end from your shoulder). The gaming term is (I believe) derived from the term 'hand of cards'. However, there are games and times when you locate your hand of cards elsewhere, like resting on the table.
As part of many games, you have a reserve of cards, often called a "deck" or a "stack." This pile is defined by the rules of the game, and the way you access the pile is defined by the rules of the game. Often, you will have a person who is assigned the role of "dealer", meaning s/he is responsible for handing out cards to each player in accordance with the game rules. Other times, you may have multiple decks/stacks, from which you will obtain cards. The act of obtaining these cards is called "drawing" them. The term "to draw" applies whether or not you are dealt cards by the dealer or if you draw them directly from the deck yourself - though individual rules of the game can change that.
"previously drawn cards in your hand" means that you have obtained cards by drawing them, and they are currently in your hand. The rule and context of the rule will indicate when those cards were drawn. Perhaps on this turn, perhaps any time since the game started.
The cards that you hold in your hand are in some order. One card may be to the left, another to the right, of that, and another to the right of that. Or they may all be held on top of each other so that you can see exactly one card. The term "arrangement" means the way things happen to be placed. It is nonspecific as to where the particular placements are - just that the specific things in question have locations in physical space. To "rearrange" is to change the arrangement in physical space. That is, to change where those things are located in physical space.
rearrange the hand
This specifies that we are looking at the locations of the cards in your hand of cards, and that you are to change the locations that they are take in physical space.
so that four cards are visible
Something is "visible" if you can see it. It is "not visible" if you cannot see it. The way that you need to set up the cards you have in your hand of cards is so that exactly four of those cards can be seen. In particular, this type of rule with card games typically means that the face of the card with interesting information on it is visible - with poker cards, you'd be able to see the number and suit of the card. However, the specific rule that you are quoting from might contain additional specification that changes exactly which side is visible.
This instruction also means that you should set the other cards so they are not visible. This could be by keeping them in your hand, or by stacking them under the other cards, or perhaps by placing them face down on the table. The specific rules of the game will provide any special instructions about how these non-visible cards should be arranged.