happen upon

VERB /hæpən_ə'pɑn/
11 LETTERS · STARTS WITH H

happen upon is a valid 11-letter word worth 19 points in Scrabble and 24 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
H4A1P3P3E1N1U1P3O1N1
4 + 1 + 3 + 3 + 1 + 1 + 0 + 1 + 3 + 1 + 1 =19 PTS
DOUBLE WORD → 38TRIPLE WORD → 57
MEANING

What does happen upon mean?