boundary line

NOUN /baʊndəri_laɪn/
13 LETTERS · STARTS WITH B

boundary line is a valid 13-letter word worth 18 points in Scrabble and 22 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
B3O1U1N1D2A1R1Y4L1I1N1E1
3 + 1 + 1 + 1 + 2 + 1 + 1 + 4 + 0 + 1 + 1 + 1 + 1 =18 PTS
DOUBLE WORD → 36TRIPLE WORD → 54
MEANING

What does boundary line mean?