Full explanation
Runway is the time you have left before your bank account hits zero, assuming nothing changes. It is your safety net as a founder.
You compute it by dividing how much cash you have by how much you spend each month.
If you have less than 6 months of runway, you are in danger zone. Most founders aim for 12 to 18 months to have time to find product market fit.
Concrete example
You have 30 000€ in the bank. Your business loses 3 000€ per month. Your runway is 10 months.