Most common
Loose gas cap. The evaporative emissions system detects pressure changes; a loose cap looks like a leak. Tighten it, drive 50 miles, light goes out.
Second most common
O2 sensor or mass airflow sensor reading marginal. Costs $30-150 to replace. The car runs fine; the light is the only symptom. You can drive on it for weeks.
When to actually worry
If the light is FLASHING (not solid) — engine misfire that can damage the catalytic converter. Pull over. Solid light = diagnose at your convenience.