The 5-Bet is the third re-raise that happens in a betting round. You can look at it as the fourth consecutive raise in a round, which almost exclusively happens in the pre-flop betting round.
For example, let’s say that you open the pot by raising from UTG, and the player in the HJ raises as well by 3-betting, only to see the BTN re-raise by making a 4-bet. Then, when the action comes to you, you decide to re-raise one more time. No matter if it’s a small re-raise or an all-in, this automatically qualifies as a 5-Bet.
