。
In short, she's a chatterbox.
Literal
She [topic-は] in-short talkative [explanatory-なんだ].
要するに ('in short, essentially') is a summarizing discourse marker introducing the speaker's conclusion. おしゃべり means 'talkative person / chatterbox.' なんだ (explanatory のだ) adds assertive tone: 'the thing is, she's a chatterbox.'