Definition
Muscle soreness that peaks 24 to 72 hours after unfamiliar or eccentric-heavy training.
It comes from micro-damage and the inflammation that follows, not from lactic acid. New exercises, slow eccentrics and a jump in volume after a break cause the worst of it.
Soreness is not a measure of a good session. It fades as the body adapts to a movement (the repeated bout effect), so a client who is never sore may simply be consistent.
Train through mild soreness and skip heavy work on severe soreness. Prevent it by progressing volume slowly and keeping exercises the same for a block.