This probably has the simple answer of "any creature that is attacking can be a target attacking creature as long as it is targetable," but I'll expand on that below.
In the Magic Comprehensive Rules, Rule 508 explains attacking creatures. It's pretty long, so rather than posting all of 508, here is the relevant part.
508.1. First, the active player declares attackers. This turn-based action doesn’t use the stack. To
declare attackers, the active player follows the steps below, in order. If at any point during the
declaration of attackers, the active player is unable to comply with any of the steps listed below, the
declaration is illegal; the game returns to the moment before the declaration (see rule 717,
“Handling Illegal Actions”).
508.1a The active player chooses which creatures that he or she controls, if any, will attack. The
chosen creatures must be untapped, and each one must either have haste or have been controlled
by the active player continuously since the turn began.
After checking for restrictions and requirements that may prevent creatures from attacking, the rule continues:
508.1j Each chosen creature still controlled by the active player becomes an attacking creature. It
remains an attacking creature until it’s removed from combat or the combat phase ends,
whichever comes first. See rule 506.4.
So creatures become attacking creatures when the steps in 508 have been completed. More often than not, there won't be requirements or restrictions that limit your ability to attack, except for summoning sickness.
When the card refers to "target attacking creature," it is referring to a creature that is an attacking creature that can be targeted. This ability can be used for one target creature. Rule 601 lays this out (I've emphasized the important parts):
601.2c The player announces his or her choice of an appropriate player, object, or zone for each
target the spell requires. A spell may require some targets only if an alternative or additional
cost (such as a buyback or kicker cost), or a particular mode, was chosen for it; otherwise, the
spell is cast as though it did not require those targets. If the spell has a variable number of
targets, the player announces how many targets he or she will choose before he or she
announces those targets. The same target can’t be chosen multiple times for any one instance of
the word “target” on the spell. However, if the spell uses the word “target” in multiple places,
the same object, player, or zone can be chosen once for each instance of the word “target” (as
long as it fits the targeting criteria). If any effects say that an object or player must be chosen as
a target, the player chooses targets so that he or she obeys the maximum possible number of
such effects without violating any rules or effects that say that an object or player can’t be
chosen as a target. The chosen players, objects, and/or zones each become a target of that spell.
(Any abilities that trigger when those players, objects, and/or zones become the target of a spell
trigger at this point; they’ll wait to be put on the stack until the spell has finished being cast.)
Example: If a spell says “Tap two target creatures,” then the same creature can’t be
chosen twice; the spell requires two different legal targets. A spell that says “Destroy
target artifact and target land,” however, can target the same artifact land twice
because it uses the word “target” in multiple places.
So the simply version of all this is that "target attacking creature" is a creature that has become an attacking creature and can be the target of a spell or ability.