Definition of bet/3bet
A bet/3bet is the act of betting with the over-raise in mind. In other words, we bet while being ready for a raise from our opponent. After our opponent raises, we go for an over-raise, which in poker is called a 3bet (the third bet at a given stage). The term is often used to describe a hand when discussing poker strategy with friends.
Some examples of sentences
If we are playing against a very aggressive player who tends to raise very often, we can say: "On the flop, I will bet/3bet my flush draw against a very aggressive villain". You can bet/3bet for value, because you have a strong hand, or bluff. Against a player who systematically raises the bets, it can be interesting to bet/3bet.
When a flop is very dynamic, often the bet/3bet should be kept 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 to bet/3bet, it would probably be best to just check and call.