To fall back means to retreat or move backward, especially when facing difficulty or danger. When soldiers fall back during a battle, they move away from the enemy to regroup and protect themselves. When a football team's defense falls back, they move toward their own goal to stop the other team from scoring.

The phrase also means to rely on something as a backup plan. If your first idea for a science project doesn't work, you might fall back on a simpler experiment you know will succeed. When a family faces hard times, they might fall back on savings they've been keeping for emergencies. A student who struggles with a math problem might fall back on the basic formulas they learned last year.

You'll sometimes hear about a fallback position, which is a safer or more reliable option you can use if your main plan fails. Having something to fall back on can give you confidence to try challenging things, knowing you have a backup ready if needed.