Yes, Netherfield's got it - to expand a little bit more - to get seed potatoes you first sow the seeds that develop on the potato plant after flowering (they are in those tomato-like fruits - beware, these are poisonous). These seeds grow into potato plants, and they grow potatoes on their roots. These are harvested and sold as 'seed potatoes' for planting and growing more potatoes. Therefore they are 'first generation' and if grown in virus-free soil are unlikely to carry any nasty viruses and as Netherfield said, can be sold as Certified.
The potatoes we buy from the greengrocer and supermarket are 'second generation' and may have picked up a virus - this will not harm us but would produce virus-infected plants if grown on and result in the failure of the crop - see references to the Irish potato famine (yes, and in other places too but because of different government policies, not with such disastrous effects ).
And Ryan, at least 85% of potatoes consumed in the UK are grown in the UK - my brother is a very large commercial potato farmer and he'd be very disappointed if it was otherwise
The majority of imported potatoes are earlies from Israel, Cyprus and Egypt which are available in the short gap when our stores of maincrop potatoes are running low and before our home-grown new potatoes are ready - the first of which are usually the Jersey Royals.