The full sentence is 「お召し上がりになってください」, which is basically 尊敬語 or honorific language. More casually, something like 召し上がってください would be equivalent.
召す is a pretty vague verb, but due to the polite way she's saying it, I'd guess "Please have your fill", which also retains a sort of metaphoric quality. EDICT lists 召し上がる as a compound specifically meaning "to eat", though, so I'm a little confused as well.
