MOD stands for modulo. The function returns the remained after a division.

The format is MOD(number to be divided, the dividing number).

This is most easily shown with an example such as breaking out hours, mins and secs from a duration:
Hours = |audience.sender.duration|/3600
Minutes = MOD(|audience.sender.duration|/60,60)
Seconds = MOD(|audience.sender.duration|,60)