How much money the company loses per month — gross burn is total spend, net burn is spend minus revenue.
The distinction matters as soon as there is any revenue at all. A company spending $20k and earning $8k has a $20k gross burn and a $12k net burn; runway is calculated from net.
Early on the largest line is almost always people, and for a solo founder that line is frequently recorded as zero — which makes the burn rate a fiction. Include what you would have to pay someone to do your job, or at minimum what you need to live on, or every downstream number is optimistic by exactly that amount.
Project types where this term stops being vocabulary and starts being a decision you have to make.