What are the meanings of 600D / 420D / 210D polyester fabric ?

Denier means the thickness of the polyester filament yarns. 

Filament yarn made from long continuous fibers. 

600D means 9000 meters yarn weight 600 grams.

420D means 9000 meters yarn weight 420 grams.

300D means 9000 meters yarn weight 300 grams.

210D means 9000 meters yarn weight 210 grams.

So yarn thickness 600D > 420D > 300D > 210D

The same denier yarns can work out different thickness of fabric, because the weaving density of yarns can be different. 

600D polyester fabric with PVC coating

420D Polyester with PU coating

300D polyester with PU coating and water proof

210D polyester with silver coating