rack up

VERB /ræk'əp/
7 LETTERS · STARTS WITH R

rack up is a valid 7-letter word worth 14 points in Scrabble and 17 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
R1A1C3K5U1P3
1 + 1 + 3 + 5 + 0 + 1 + 3 =14 PTS
DOUBLE WORD → 28TRIPLE WORD → 42
MEANING

What does rack up mean?