Beginning knitters often work with patterns that don't include any shaping, that is, the number of stitches stays the same all the way across the work. That's fine for scarves and afghans, but if you want to make a sweater you need to be able to change the number of stitches, which changes the shape of the item you are knitting.
To get more stitches, an increase is needed. The most basic way to increase is knitting in the front and the back of a stitch.
First knit in the front of the stitch that needs to be increased, like you normally would. But instead of sliding the right-hand needle to the front and taking the stitch off the left-hand needle, you tilt the right-hand needle so you can work a knit stitch in the back of the loop of the same stitch. Then you can slide the two new stitches off the left-hand needle and proceed with the pattern.
