Surge means that there are more users ordering rides than there are drivers, so the price 'surges' to encourage more drivers into the area. Depending on demand you can earn up to 3 x the normal fare through surge.
Surge applies to the whole fare, including the Central London Fee and waiting fees. You will also receive surge pricing on cancellation fees.
Be aware that the fare will be based on the surge at the pick-up location rather than the location from which you accept. For example, if you are in a 2.5 x surge zone and the customer is ordering from a 2.2 x surge zone, the fare will be multiplied by 2.2 x.
You can see where surge is at any point in time through the main map, by zooming in you will see the exact surge amount in that area, as below: