composition board

NOUN
17 LETTERS · STARTS WITH C

composition board is a valid 17-letter word worth 25 points in Scrabble and 30 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
C3O1M3P3O1S1I1T1I1O1N1B3O1A1R1D2
3 + 1 + 3 + 3 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 0 + 3 + 1 + 1 + 1 + 2 =25 PTS
DOUBLE WORD → 50TRIPLE WORD → 75
MEANING

What does composition board mean?