My god, thank you. I remember as a kid this explanation "the top has to move fast to meet the bottom stream at the trailing edge" always confused the hell out

My god, thank you. I remember as a kid this explanation "the top has to move fast to meet the bottom stream at the trailing edge" always confused the hell out of me. Why do the top and bottom have to meet? How would they know? Even as a 10 year old it confused me no-one could answer this.