A London Gap is a kicker or bank with a jump to a higher platform. In between, there is a lower flat surface which is gapped.
London Gaps are rare in street skating. However, you can often find them in skate parks. If you feel safe and confident enough, you can Ollie over the gap with a variety of flat tricks.
Best practice trick:
To bridge the London Gap, you have to have already mastered the Ollie. If you feel safe enough to Ollie a gap, try a Pop-Shove-It over the London Gap. This video will teach you how to do just that: