Yes, TUMBLES is a Scrabble word!

Letter values in Scrabble US: T 1, U 1, M 3, B 3, L 1, E 1, S 1.

Is it a valid word?

TUMBLES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

fall down, as if collapsing
cause to topple or tumble by pushing
roll over and over, back and forth
fly around
fall apart
throw together in a confused mass
understand, usually after some initial difficulty
fall suddenly and sharply
put clothes in a tumbling barrel, where they are whirled about in hot air, usually with the purpose of drying
suffer a sudden downfall, overthrow, or defeat
do gymnastics, roll and turn skillfully

Also: whirl around, break down, catch on, get wise, get onto, latch on, cotton on, get it.