A notch is a small cut or nick in something. People make notches to keep track of things.
If you've even seen a little U-shaped or V-shaped cut in something, you've seen a notch. In prison, criminals will make notches on the wall to keep track of how long they've been there. If you're stranded on a desert island, you might make notches on a tree. When you make a notch, you're notching. A notch is also part of a mountain range that is lower than the peaks around it — also called a mountain pass.