Acetate is a type of plastic - used in cooking it is generally a thin plastic sheet that is flexible basically nonstick - I don't know that there is a specific "food grade" acetate (although as soon as I say that someone will find a source for one).
No - you don't eat it. It's a "tool", not an ingredient. It's kind of like when you make a loaf of bread you don't eat the pan ... same thing with acetate.
For working with chocolate - you can either use it flat or roll it into a tube to make various sized forms. You can also use it in proofing bread - to increase the height of a round loaf - you just make a collar and put the dough into it to proof.
You can find this at most stores that sell supplies for candy making, maybe pastries, or an art supply store.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain