The two blank tiles included with Scrabble sets have no letters on them. According to the box and official tournament rules, they may be used as a "wild card" to represent any letter the player declares when they are played.
A popular rule variant commonly found in informal games is to allow a blank tile that has been played to be swapped back into the player's inventory when they obtain the letter that the blank tile had been representing. This rule is not included in the box rules and is not allowed in official Scrabble tournaments. Some "house rules" players also prefer to remove the tiles entirely from the game.