The word lap has several meanings:

1. The flat area formed by your upper legs when you sit down. A cat might curl up in your lap, or you might balance a book on your lap while reading. When you were younger, you probably sat in a parent's or grandparent's lap to hear a story. The lap disappears when you stand up; it's just the surface your thighs create when you're seated.
2. One complete circuit around a track or course. Runners completing a lap around the track return to where they started. In a swimming race, a lap means going from one end of the pool to the other (or sometimes there and back, depending on who's counting). Race car drivers complete many laps during a race, circling the same course over and over. When someone laps another racer, they've gone around the track one more time, actually passing them despite being “behind” in position.
3. To drink by scooping liquid with the tongue, the way a dog or cat drinks water. Animals lap up milk or water from a bowl. You might see waves lapping at the shore, touching the beach repeatedly with a gentle, rhythmic motion similar to a tongue.