A POT still simply collects and condenses the alcohol vapours that come off the boiling mash. This will result in an alcohol at about 40-60% purity, with plenty of flavour in it. If this distillate were put through the pot still again, it would increase in purity to around 70-85% purity, and lose a bit of its flavour.
A REFLUX still does these multiple distillations in one single go, by having some packing in a column between the condensor & the pot, and allowing some of the vapour to condense and trickle back down through the packing. This “reflux” of liquid helps clean the rising vapour and increase the % purity. The taller the packed column, and the more reflux liquid, the purer the product will be. The advantage of doing this is that it will result in a clean vodka, with little flavour to it – ideal for mixing with flavours etc.