When many people are booking in an area and few captains are free, a surge multiplier applies to fares there. It's what makes it worth a captain's while to head into the busiest areas, so that rides keep being available when demand is highest.
Surge is always shown before you confirm — you'll never find out about it afterwards. If you can wait, fares usually settle within a short time.
Surge is capped. Each city has its own limits — by default a fare is never more than twice the normal fare — and where a state sets a lower limit, we keep to it. Surge never applies to waiting time, airport fees, tolls or parking.