An increase in knitting (abbreviated as “inc”) is a way to add stitches. There are many ways to increase. Unless the pattern specifies a specific increase, you can use the method you prefer.
Increases can be right-leaning or left-leaning, made by yarn overs, knitting into the front and back of stitches, lifting stitches, lifting yarn, and a combination of these methods. Master a few increase methods so you have more than one to choose from.
The most common increases are: the make one (or lifted bar increase), the kfb (knit front & back), and the yarn over.
Inc 1st vs Inc 2sts or more
Most methods of increasing have you increase one stitch. There are increases that increase more than one stitch, such as the double yarn over, the kfbf (knit front, back, front), and the (k1, yo, k1, yo, k1) in one stitch. Lare increases are primarily used in more advanced lace patterns, making bobbles, and complex shaping.