Burn rate is the startup-world term for a company's monthly cash outflow. Gross burn is simply total monthly operating expenses — payroll, rent, software, everything. Net burn subtracts whatever revenue came in that month, and is the number that actually determines how fast the bank balance is falling.
A profitable, revenue-generating company can have a high gross burn but zero or negative net burn, meaning cash is actually growing. A pre-revenue startup's gross and net burn are usually identical, since there's no revenue to offset spending.
Investors watch burn rate closely because, combined with cash in the bank, it determines runway — how many months the company can operate before running out of money. A sudden, unexplained jump in burn rate is one of the first warning signs investors look for in board updates.