Yes, EXPOSITION is a Scrabble word!

Letter values in Scrabble US: E 1, X 8, P 3, O 1, S 1, I 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

a systematic interpretation or explanation (usually written) of a specific topic
a collection of things (goods or works of art etc.) for public display
an account that sets forth the meaning or intent of a writing or discourse
(music) the section of a movement (especially in sonata form) where the major musical themes first occur