"Out of a glass"
Translation:Aus einem Glas
"Aus" carries the idea of "of" with it, so adding a "von" would be redundant. There's a lot of these slight differences between English and German that just have to be learned.
As another example, "suchen" means "to search for"; the idea of for is already included in the idea of suchen. So the sentence "Ich suche für meine Katze" would be wrong because of the redundant "für." You just have to say "Ich suche meine Katze."
That would probably be "from a glass" -- though without context it's difficult to say for sure.