Ribbing is often used to make things like collars, cuffs or the rim of a knitted beanie – ribbing makes your knitting a bit tighter and more elastic.
Tips and instructions for ribbing in knitting:
Knit one stitch as you usually would.
Before you start your next stitch, move your loose wool that you are knitting with to the opposite side of your work. Now you are ready to make a purl stitch.
Purl this next stitch and then move your wool back to create a knit stitch.
NOTE: Ribbing isn't always just Knit one, Purl one. Your pattern may have a rib such as K2, P2 or even K1, P2, etc.