Woven fabric consists of warp and weft yarns crossing each other one at a time or in groups. Plain weave always consists of one warp yarn crossing one weft yarn, a 1/1 weave. When two warp yarns cross a weft yarn, this can be indicated as 2/1 weave. 2/2 weave has two warp yarns crossing two weft yarns.
These fractions are read, for example, “three up, one down” for 3/1, indicating that three weaving harnesses are raised, then one is lowered for three warp yarns on the face, then one weft yarn.