The letter "D" stands for Denier, which is a numbering system used for fibers, filaments, and yarns. In this system, lower numbers indicate lighter/finer materials, while higher numbers indicate heavier/coarser materials. On the other hand, "T" stands for Thread Count, which refers to the number of warp and fill threads in a square inch of fabric. Lower thread counts represent loosely woven fabric, while higher thread counts indicate a tightly woven fabric. Together, these two numbers provide an indication of the strength and feel of a particular fabric.