何を～ますか？ / What do you do?

何

"何(なに) / what". 

何を

"を" is put after "何", so that you mark "何" as the object.

何を食べますか？

"食べます（たべます）/ eat". this sentence means "What do you eat?". As in Enlgish, "What do you eat?" is asking "what do you eat usually/normally/regularly".

まいあさ　なにを　たべます　か？

This is the example that you can understand clearly for the use of this grammer. "まいあさ / every morning". The sentence means "What do you eat every morning?".