How to remove exposed aggregate patio
Aggregate is the gravel and sand that is mixed with cement and water to make concrete. so the surface you see is the concrete of the patio.

Exposed aggregate is prepared by applying a chemical to the surface of fresh concrete which slows the hardening of the top layer.  when the bulk of the concrete is hard the top cement is hosed off exposing the aggregate. (the run-off makes a real mess)

You've got a few options to resurface. you could use a skim coat of mortar slurry  with some additive to improve bonding resulting in a classic concrete looking surface. or a leveling compound. It probably wants a good cleaning before you try to bond anything to it.

Or you could grind it down to get the terazzo-like look of cut concrete 

Or you could tile it.