Tight
A tight player plays few hands, and mostly strong ones. He folds many hands before the flop, and he is often very selective after it. The opposite is a loose player.
The simple explanation
A tight player waits for good cards. When he bets or raises, he usually has a strong hand, so you should be careful against him. You can win his blinds because he folds too much.
A closer look
Tight is a description of how many hands a player chooses to play, and it does not say how aggressively he plays them. A tight-aggressive (TAG) player plays few hands but raises them, and a tight-passive player calls a lot. Tightness is good in moderation: a very tight player, a nit, is easy to read and loses money in the blinds. The right level depends on the game: in tough games, you should be somewhat tight, and in soft games you can play more hands.
A practical example
A player folds for an hour and then raises. The others give him credit for a strong hand, because he is tight.
Full explanation
A tight player plays few hands, and mostly strong ones. He folds many hands before the flop, and he is often very selective after it. The opposite is a loose player.
A tight player plays few hands.
He plays mostly strong hands.
The opposite is a loose player.
A game with little action can also be called tight.