How long your capital lasts
Runway is the number of months your capital will last at your current net spend. For a new business that is the single most important number, because it sets the clock you are racing to reach break-even or your next funding.
As your income grows, your net monthly spend shrinks and the runway extends. Watching it every month tells you whether you are pulling away from the cliff or heading toward it, while there is still time to change course.