Whether you're talking about a 600D polyester tote bag or a 210D nylon lunch cooler, the D stands for denier which is a unit of measurement describing the linear mass density of the material, calculated by the mass in grams of a single 9,000 meter strand (reference: one denier is the mass of one 9,000 meter strand of silk).
Denier is a good measure of strength and durability when you're comparing two deniers of the same material. For example, a 400D nylon bag is not as strong as a 600D nylon bag. However, when you're comparing between different materials, such as nylon and polyester, all deniers are not equal. Since nylon is a stronger material than polyester, 420D nylon is actually stronger than 600D polyester.