What does sitting on his lap mean?

So one day I was laying next to my boyfriend. Then he grabbed my arm and pulled me on top of him. So I was sitting on top of him, he was in between my legs. We were kissing and then all of a sudden he started adjusting himself. Then he asked if I understood, I explained I had no idea no idea what he was talking about. Then he just said never mind your to young but me and him are only 4 days apart. So does anyone know what happened?
What does sitting on his lap mean?
Post Opinion