law of action and reaction

NOUN
26 LETTERS · STARTS WITH L

law of action and reaction is a valid 26-letter word worth 33 points in Scrabble and 39 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
L1A1W4O1F4A1C3T1I1O1N1A1N1D2R1E1A1C3T1I1O1N1
1 + 1 + 4 + 0 + 1 + 4 + 0 + 1 + 3 + 1 + 1 + 1 + 1 + 0 + 1 + 1 + 2 + 0 + 1 + 1 + 1 + 3 + 1 + 1 + 1 + 1 =33 PTS
DOUBLE WORD → 66TRIPLE WORD → 99
MEANING

What does law of action and reaction mean?