How many months of cash you have left before you run out. Your oxygen.
Runway = cash-on-hand ÷ monthly burn. It's the simplest calculation in startup finance and the one you should know cold at all times. Healthy runway is 18+ months; you have time to experiment and build. Tight runway is under 6 months; every decision is about survival.
Runway is the clock every startup lives by. You want to either reach profitability or raise your next round with at least 6 months still on the clock. Everything that isn't on the critical path of doing that is a distraction.