The serpentine belt drives multiple accessories from the engine crankshaft: alternator, water pump, power steering pump, AC compressor, sometimes the air pump or other systems. One belt, multiple jobs.
Belt failure cascades fast. The alternator stops charging — the battery starts draining. The power steering pump stops — steering gets heavy. The water pump stops — engine overheating begins. AC stops working.
Warning signs before failure: squealing noise especially when cold-starting, visible cracks or fraying when inspected, glazed appearance on the belt's ribbed surface, mileage past the manufacturer's replacement interval (often 60,000–100,000 miles).
If the belt breaks while driving, you have a few miles to find a safe place to stop. Don't try to drive home — the engine will overheat and cause much more expensive damage.
Replacement is one of the cheaper repairs: $80–$200 at a shop including parts and labor. Worth doing preemptively at the manufacturer's recommended interval.