Ribbing patterns are columns of knits and purls alternating each other. These patterns tend to pull in, making them great for collars and cuffs of jumpers or hats. Ribbed fabric does not roll at the edges, making it very suitable for scarves as well.
You can create all types of ribs with different amount of stitches per column per side. Popular examples are 1x1 rib (as shown below), 2x1 and 2x2 rib.