A cadence is an eight-count marching or running song, used in military type training. The formation leader calls out the song (starting on the left foot) and the unit keeps pace. Each line should also finish on the left, the right foot comes down during the pause. The "left" foot words/syllables are often slightly emphasized. Thus we have
LEFT-right-LEFT-right-LEFT-right-LEFT,
LEFT-right-LEFT-right-LEFT-right-LEFT (and so on).
Cadences help set the pace, enforce proper breathing, and build unit morale. For a sample, see airborne.
It might be tricky to call out during some of the activities listed above.