Increase is a shaping technique which leaves you with more stitches than what you started with. There are numerous ways to do this. You can increase a stitch by 1 or 2 or however many stitches you need. In patterns you will either get clear instructions on which kind of increase you need to make and/or how many times you will be increasing. When patterns ask to increase your stitches across the whole piece evenly you need to divide your stitches by the number of increases. This will give you a stitch number where you make your increase.