A per diem is a daily allowance paid to each band and crew member to cover meals and incidentals while they're on the road. It's usually a set amount per person, per day, and it applies to every day of the tour, including travel days and off days, not just show days.
Because it's paid out to so many people so often, per diem is one of the easiest things on a tour to lose track of. A clear record of who was paid, how much, and when keeps the touring party happy and keeps your books clean.