Increasing (inc 1)
Increasing the number of stitches on your needle is a way to either make your knitting wider, or to create a different shape to your knitting. It's used on all my knitted birds wings, to create a smooth, curved wing shape.
There's many ways to increase in knitting, but the only one that I use in my patterns is by knitting front and back into the same stitch (abbreviation inc 1 in the knitting patterns).
To increase a stitch using this technique is very similar to making a knit stitch, only you don't drop the stitch off your needle, you knit into it again. Here's a helpful, slow motion video by Very Pink Knits of the technique:
When you see inc 1 in a pattern, use this increase in the stitch that you are working to turn 1 stitch into 2. When you see inc 2, use this increase in the next 2 stitches that you are working to turn 2 stitches into 4.
As an example, have a look at Barry the Puffin's wing. You start by casting on 6 stitches and then the first row is:
Row 1: k1, inc 1, k2, inc 1, k1 (8 sts)
So you make a knit stitch in the first stitch, then increase into the next stitch, then knit 2 more stitches, then increase into the next stitch, and finally knit one more stitch. Because you've made 2 increases, your stitch count has gone up from 6 stitches on the needle, to 8 stitches.