The third bet in a betting round before the flop: the big blind is the first, an opening raise is the second, and a reraise over that open is the third. Four-bet and five-bet continue the same count.
A three-betting range has to do two jobs at once. Some of it is value — hands strong enough to want more money in against the range that opened. The rest is pressure, hands played this way precisely because they do not want to see a flop as a call. Built only from premium holdings, the move becomes transparent: an observant opponent folds everything except the hands that beat you, and your best cards win the smallest pots available to them.
That is why the bluffing half is usually made of hands with some way of continuing when called — suited aces, suited connectors, hands that make strong draws and can put real pressure on later streets — rather than the weakest cards in the deck. A hand that folds the flop every time it misses is a poor candidate; a hand that can turn into a genuine threat is a good one.
Sizing follows position. In position a smaller reraise is enough, because the positional edge does the rest of the work. Out of position the price should be higher, both to fold out hands that would otherwise realise their equity cheaply and to avoid playing a bloated pot with the whole hand still ahead of you and every decision to be made first.