Yes, HEADER is a Scrabble word!

Letter values in Scrabble US: H 4, E 1, A 1, D 2, E 1, R 1.

Is it a valid word?

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

a line of text serving to indicate what the passage below it is about
horizontal beam used as a finishing piece over a door or window
brick that is laid sideways at the top of a wall
a framing member crossing and supporting the ends of joists, studs, or rafters so as to transfer their weight to parallel joists, studs, or rafters
a machine that cuts the heads off grain and moves them into a wagon
(soccer) the act of hitting the ball with your head
a headlong jump (or fall)