Not a daft question at all, as we can't all be expected to translate marketing/industry speak. 'Sell through' refers to non-rental discs which are available at retail to the public. As with VHS tapes, rental versions differ from those the end consumer is able to buy from a shop (sell-through).
The sell through. It's what you see for sale in all the shops and on web sites. Rental disks normaly have no extra's an cost between £30 to £60. It does mean if you bought a rental disk, you could rent it out to family and friends for money and not be breaking the law though.
I've always thought it's rather silly that Splash DVD advertise some of their DVDs as "sell throughs". We would presume that they were regular retail DVDs anyway. If they have rental versions for sale (which I've never seen, but then I've never looked hard enough for them), then it's better to just mark up those ones as being rentals and stop confusing everybody!