Burn rate is the amount of money a company spends each month. Gross burn is the total sum of money a company spends. The net burn is the gross burn excluding the revenue. E.g. a company spent $ 30 000 last month (salary, hosting, etc) and got revenue $10 000. So, company gross burn is $30 000 and net burn - $30 000 - $10 000=$20 000.
At the early-stage startup companies don’t have any revenue, so, the gross burn is equal to net burn.
It is also possible to find out the runway of a startup company. E.g. an early-stage startup company has $500 000 in a bank, and its gross burn rate is $50 000 and it is equal to net gross. So, the company runway would be $500 000 / $50 000 = 10 months, if no revenues.