When a player is on "Tilt" it just means they are playing poker differently than they normally would, ie worse. Usually it is meant in a negative way, for example when a player takes a bad beat in a tournament and then shoves all his chips in the middle without caring what his cards are or anything else, that is called a Tilt shove.
There is winners Tilt too, when a player has a huge stack they will often start making very loose calls and spew chips because they are ahead, that is also Tilt.