Yes, EMPHASIS is a Scrabble word!

Letter values in Scrabble US: E 1, M 3, P 3, H 4, A 1, S 1, I 1, S 1.

Is it a valid word?

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

special importance or significance
intensity or forcefulness of expression
special and significant stress by means of position or repetition e.g.
the relative prominence of a syllable or musical note (especially with regard to stress or pitch)