A notch is a small cut in the fabric (or a triangle on some commercial patterns) that help guide you while you are sewing. They are used to indicate seam allowance, dart arms, the location of design details, such as pleats, tucks, gathers, hems and pockets, or indicate key points on the pattern (like the centre front or centre back). Notches are also used to indicate balance points (points on your pattern that help you sew the right pieces together, as well as help you when you are sewing long or curved seams).
When adding notches to a pattern, try to avoid notching both sides of a corner - this can weaken both the pattern and the fabric.
Just be methodical when notching. If the sides seam allowance is marked with a notch at one end, it doesn't need to be done at the other. At the other end of the same seam, you may use a notch to indicate the hem or seam allowance for the connecting piece.