Some ballet companies are lucky and own a theatre that they use all of the time to rehearse and perform in, but the majority of ballet companies in the world must use another theatre because they don't have the money to buy or build one.
When the dancers arrive in the theatre for the first rehearsals, quite often all that has been done is a dance floor will have been laid down and some lights will have been hung. During the course of the in-theatre rehearsals the theatre will be prepared for the show. The theatre is not prepared ahead of time because renting a theatre is very expensive. Once they are dancing in the theatre the dancers will get used to the new floor, the new size of the stage, and figure out where to make their entrances and exits, where to look when they are dancing, and so on. These rehearsals are called blocking rehearsals.
The second set of rehearsals that happen in the theatre are the technical rehearsals. This is when the ballet is run from start to finish with all of the lights and sets. The purpose of the technical rehearsals is to let the dancers get used to the lights and to let the stage crew and lighting operators practise doing their jobs. Finally there is the dress rehearsal. This is usually the evening before the show opens and is run in full costume, full make-up, and is just like a performance without an audience. A dress rehearsal is to go over the show full out and make sure that there will not be any major avoidable problems. The orchestra will be at the dress rehearsal playing the music for the dancers. This may be the first time that the dancers work with the orchestra so they must pay careful attention. Finally, the dress rehearsal allows everyone to get used to working in performance conditions.