Passing the action to the next player without betting.
You can check only when no one has bet yet in that round. Example. If the action reaches the BB with only limpers, the BB can check and see the flop at no extra cost. It is written x and often appears as part of a line, as in x/c (check-call).
At the table
On the flop, the first player to act says "check" or taps the table, and the action moves on without a bet. If everyone after them also checks, the turn card is dealt.