。
She's a great speaker.
Literal
She [topic-は] public-speaking [subject-が] skilled.
The XはYが construction expresses an attribute of the topic: she (X) — when it comes to speaking (Y) — is skilled. This double-marker pattern is fundamental and powers descriptions like 私は背が高い ('I am tall' / 'as for me, height-wise, am tall') and 象は鼻が長い ('elephants have long trunks'). うまい is a casual cousin of 上手, both meaning 'skilled/good at.' うまい also means 'tasty,' so context disambiguates — here it can only mean skilled because the subject is an activity (演説), not food. Note that with skill expressions, the activity takes が, never を, even when it would be the object of the underlying verb.