Definition of re-raise
Preflop, when you pay on the blind, you are said to be filing. If you raise to 2 or 3 or 4 big blinds, we'll say you raise. And if a player raises your raise to say 10 big blinds, we'll say he makes a re-raise. This game is also called a 3bet. You can make a re-raise because you have a very good hand or because you want to steal from the raising player. For example, if a player makes a steal (raise the cutoff or the button), a player in the small blind position may decide to make a re-raise (resteal or 3bet) to win the pot.
Re-raise and 3bet
It is easy to understand what a re-raise is. But it is not necessarily the vocabulary that will be used the most. At the poker tables, with your friends and even on the trackers, you're more likely to find the expression 3bet, which represents the 3rd bet (bet, raise and re-raise). If a player raises a re-raise, it will be called a 4bet. As a general rule, a 4bet is polarized, i.e. you do it either by bluffing or because you are very strong.