"We like the idea that she went to the museum."
Translation:Nous aimons l'idée qu'elle soit allée au musée.
It is the third person conjugation of the verb être in the subjunctive mood
The subjunctive mood is used to express actions or ideas which are subjective or otherwise uncertain: want, emotion, doubt, possibility, necessity, judgment. It is nearly always found in dependent clauses introduced by que or qui, and the subjects of the dependent and main clauses are usually different.