。
She has a tremendous appetite.
Literal
[topic-は] tremendous appetite [subject-が] exists.
ものすごい ('tremendous, terrific, astonishing') is an i-adjective stacking もの ('thing') + すごい ('terrible, amazing') — the prefix もの intensifies すごい to its strongest reading. The pattern Xがある describes possession or existence of an attribute — 食欲がある ('has an appetite,' lit. 'an appetite exists'). This がある-marked existence pattern is one of the most basic Japanese ways to attribute a feature to someone.