Warp and weft yarns are combined in different ways to produce weave designs. These designs affect the appearance, feel, strength, and durability of the fabric.
The simple warp face designs used in denim are designated by the number of weft yarns that the warp ends pass over followed by the number of weft yarns they pass under.
The most common weaves in denim are three over one (3×1), two over one (2×1), and three over one broken twill. The 3×1 and 2×1 come in either left or right hand direction. The most common design in denim is the 3×1 right-hand twill.
Source: Tellason Denim Terminology