Definition of term bet/3bet
A bet/3bet is the action of betting with the over-raise in mind. In other words, you bet while being ready to raise your opponent. After our opponent's raise, we go for an over-raise, which we will call 3bet to poker (the third bet at a given stage). The expression is often used to describe a hand when discussing poker strategy with friends.
Some examples of phrases
If we play against a very aggressive player who tends to raise very often, we can say: "On the flop, I'm going to bet/3bet my color draw against a very aggressive bad guy".
You can bet/3bet in value, because you have a strong hand or in bluff. Against a player who systematically raises, it can be interesting to go for a bet/3bet.
When a flop is very dynamic, often, you have to have the bet/3bet in mind. A good example is when you have a hand like 9-9 on a flop like 9-J-Q with two hearts. Here you know that your bet will often be raised on this type of texture. When a player bets on a dynamic flop and is not ready for a bet/3bet, it would probably be better to just pass (check) and call.