The term "Scrabble rack" refers to the wooden pieces that come with the board game Scrabble. They're designed to hold the lettered tiles used in the game. A Scrabble set typically includes four Scrabble racks.
Scrabble rack may also refer to the seven letters held by each player during the game, in addition to the holding rack. In this way, the term rack can also be used in electronic versions of the Scrabble game which lack a physical rack. When Scrabble players refer to a "bad rack" or a "good rack," they're most likely speaking about the quality of the letters they have. A bad rack may be a rack with all vowels.