Net burn is the change in cash balance per month. It's the most operationally meaningful burn metric because it's what divides cash on hand to compute runway. A company with $10M in the bank and $500K monthly net burn has 20 months of runway; the gross burn might be much higher but is offset by revenue.
Net burn can swing materially with large customer payments, refunds, or one-time fees. Smoothing across multiple months gives a more honest picture.