Sometimes a composer wants a pause on a particular beat. They indicate this using the fermata symbol. Not all fermatas have the same duration. The length is governed by various factors including text, tempo, character, harmonic implications, where it occurs in the bar, etc.

For the conductor, showing a fermata involves four things: preparing the choir (one beat in advance), stopping the sound, holding the pause, and then re-starting the sound. As a general rule, the duration of any fermata should be at least as long as the printed value, plus a bit more.