Definition of check
The check (or pass) is one of the most common actions in poker. As the name implies, it means that we pass, that we give the floor to the next player. In order to pass, no bets must have been made beforehand. For example, I'm the first player to talk to flop and no one has bet yet. I can bet or check to pass. If a player was before me and had bet 5 euros, I wouldn't have the option of check. It's the same principle as preflop. The only player who can check preflop is the big blind if nobody raises.
The reasons for check
We can check for several reasons. And the fact of check does not make us lose our turn. You can check in position to play a smaller pot. We can check in the idea of giving up our hand in the future because our hand is too poor. You can check to make your opponent think that your hand is bad and to make him bluff you. You can check instead of betting because you have a good hand and your opponent is still betting after your check. So, instead of betting, say, 5 euros and having our opponent simply call the bet, we can check, let our opponent bet 5 euros and then raise to 15 euros (here, a check-raise). We will then play a bigger pot with a strong hand. The reasons for check are numerous.