An increase is a way of creating more stitches on the needle.
There are many different ways to increase – a yarn over (YO) makes a decorative increase called an eyelet.
Knitting into the float between two stitches, often called a raised bar increase, basically does the same thing as a YO. Twisting the float before knitting into it makes a more invisible increase.
The method below, which I call the Grandma Increase, makes the most invisible increase of all when worked in stockinette stitch.
Tip: When increasing evenly on both sides of a piece, as in a sleeve increase, work the increases inward from the edges of the knitting in two different rows.
An easy rule of thumb is to work increases on the right side of a piece on a RS (right side) row, and work the increases on the left side of the piece on a WS (wrong side) row.
Wrap the yarn around the right needle and pull the strand of yarn through this stitch, creating a new stitch on the right hand needle.
Knit the stitch above the increase (the mommy stitch).
This will make a right-leaning increase.
For a left leaning increase, work the mommy stitch first, then knit into the grandma stitch (this is harder to do).
To work this increase on a wrong side row, purl into the stitch below the next stitch on the needle (grandma purl).