When a choir or orchestra sees the words a tempo in their sheet music, they know they should go back to the original speed in singing or playing. This instruction comes after a faster or slower passage in the music.
Tempo is the Italian word for "time," but in music it means "pace" or "speed." The word has entered English, so you might hear sentences like "As we were pedaling home it started raining, so we picked up the tempo." In a tempo, the a is the Italian word for "to" or "at," and the "tempo" referred to is the original or usual tempo of the whole piece. So a tempo means "at the original speed."