Edges are the little hairs on some (not all) girls hairlines that are smaller than your longer hairs. Girls, mostly black girls, lay their edges with gel or edge control. But dont be laying down yo adult hairs, them is not baby hairs.
It’s a style you do for the baby hairs typically a “swoop” on your upper forehead. People mostly use edge brushes but you can also use a toothbrush (only for black/light skin)