Suppose my RHO opens one of a minor. I can "overcall" in a major suit with a five card suit and a minimum of seven or eight high card points, provided that those points are concentrated in my suit. That's true even though my hand is weaker than my opponent's opening hand. Here I can plausibly fight for a part score (or a sacrifice) because I have a higher ranking suit to offset my weaker hand (depending on what each person's partner has).
If RHO bids one spade, I have to overcall at the "two" level. Since I'll have to take an extra trick, my guess is that I will need an extra trick (king) to overcall, bringing the required strength of my hand to 10-11 high card points), close to opening hand level, as well as a reasonably good five card suit.
But most of the literature I've read suggests that the above is not enough; that to overcall at the two level, one needs a six card suit* or a very strong five, say AKJxx or AQJTx (AQJxx is not good enough), and/or 12-13 high card points. Is this greater strength (versus the previous paragraph) necessary for a two level overcall, and if so why?
*On the other hand, my understanding is that if you are making a "jump" overcall with six cards (in a sequence of one minor, two spades), you can do so with strength approaching a "weak two" bid.