The simplest way to explain 3-bet and the most common way to talk about it is as the first re-raise before the flop. When someone raises to start the pot, and you raise again, this is called a"3-bet."
Even though the term"three-betting" is also used after the flop, when you hear the name"3-bet," it usually means a spot before the flop.
To make it even more straightforward, think of the blinds as the first bet, the first person who raised as the second bet, and the person who re-raised as the third bet.
For example, say you are sitting on the button with KK in a $2/$5 cash game, the hijack raises to $15, and everyone folds. Since you have a strong hand, you decide to re-raise, which is called a"3-bet."
This move can help you win big pots when you have strong hands, make your opponent fold when you are bluffing, or get rid of other players so you can play a heads-up pot.