It is a dialectical form of the preposition which means "facing", as in: "The house sat ACROSS from the schoolyard", or "The house sat FACING the schoolyard".
Acrost is not correct. You will never, in your lifetime, find it in a dictionary, and you should never use it. I will repeat: YOU WILL NEVER FIND "ACROST" IN A DICTIONARY. People who use the preposition "acrost" as a replacement for "across" are usually ignoramuses and should be ignored.
Acrost, is, of course, completely incorrect. One should say:
We ran ACROSS the parking lot.