When we talk about "tilting" in poker we refer to the way in which a bad beat can affect the game of a player. If a player suffers a bad beat and changes the way in which they usually play, we say the player is "going on tilt".
In a few words, "tilting" describes how a player starts playing poorly at a poker table because he is frustrated.
Bad beats are not the only things in a poker game that may set a poker player on tilt. When you don't get good hands for a long period of time you can also feel irritated and play with any poor hand you receive.