What does Sizing mean?

Applied to reinforce warp yarns during weaving.

Heddels explains Sizing

During the sizing process, a protective coat is applied to the fibers to smooth and strengthen them during weaving. The type of fiber and yarn thickness determine which sizing solution is used, though it must be some sort of water soluble polymer.

Common sizing agents include starch, PVA, acrylate, and wax. Sizing is done solely to prepare the fibers for manufacturing and is later removed during the desizing process.

Additional Resources

Vintage photo of warp being sized: