In fashion the notch is the cut out section on a mens suit. It separates to collar from the lapel. The term "High-Notch" refers to the location of the notch on the jacket.
The more buttons on a suit the higher the notch will be. A four button suit for example requires the notch to be almost at the height of the collarbone. Otherwise the lapels would be too short. For a two button suit a high-notch lapel should be combined with a narrow lapel and narrow collar - a look that slims down the line of the suit.