In a theatre, the orchestra pit is usually just in front of, or underneath the stage. During a performance, this is where the orchestra will be. The conductor will normally stand at the front of the pit, so that he can conduct the orchestra (below him), and the singers (above him). If he is not in a position where the singers can see him, there can often be a video camera set up, with monitors either at the front of the stage, or in the auditorium.

It a lot of theatres, the orchestra pit can be covered for shows where an orchestra is not needed - this can give extra space on stage, or just in front of it.

If you're after any other theatre tech information, then do take a look at 'Everything you ever wanted to know about theatre tech, but were afraid to ask'