The bikini gap is when a girl has hip bones which protrude out further than her flat tummy. So when she is wearing bikini bottoms or panties the waist band rests only on the hip bones and creates a visible gap between the waistband and the stomach. This is typically enhanced when she is laying flat on her back.
This amazing spinner set me a phone pic of herself looking down her at her bikini bottoms. Her bikini gap was totally hot!