what does ''about face'' mean?

why did he call the album like this, and what is he doing on the front it almost seems as if he is playing guitar without a guitar in his hand

This is totally speculative, but since an "about face" is a drill command/move for reversing the direction you're facing, I'd say Dave picked it as a symbol for his solo effort being 180º from his efforts with Pink Floyd. Who knows. Ask Dave.

His hand, on the album cover, is not simulating 'playing guitar'; it's more like a hitchhiker's gesture. He's looking to his right, but gesturing to his left. 180º....hmmmmmm.....?

my error sorry

nice theory there but here's another one. roger's pros and cons, to it was released 2 months earlier than about face, shows a nude hitchhiker wich we don't see the face. i always thought, by dooing a hitchhiker moove, david was asking her to turn around ... just another theory

That makes sense. "About Face", baby!