。
She wants to go to the party.
Literal
She [topic-は] party [to-に] want-to-go [observable-がる] [is-ている].
~たがる is the third-person desire suffix: where ~たい is fine for the speaker's own wants, attributing wants to another person normally requires ~たがる ('shows signs of wanting') or hedging like ~たいようだ. This is because Japanese is reluctant to make confident claims about another mind's interior — desires must be inferred from observable cues. The ~ている then marks an ongoing state of that desire being visible. Compare with 行きたいと言っている ('says she wants to go'), which would explicitly cite her speech.