Rib knits are knit with a rib stitch which forms wales of knit that are alternating from one side of the fabric to the other. This makes for a very stretchy, comfortable knit that hugs close to the body. This capacity makes for clothing that can fit a wide range of shapes. 2x1 is another name for a variation of Rib Knit in which the wales are knit 2 on one side and 1 on the other.