A footprint groundsheet is a groundsheet that sits between the ground and any Outwell tent that has a sewn-in groundsheet. These Outwell footprint groundsheets are cut to the shape (footprint) of the tent and so when laid out on the floor act as a useful 'marker' as to where your tent should be pitched.
A footprint groundsheet protects the sewn-in groundsheet of your tent and greatly reduces the chances of the sewn-in groundsheet being punctured by something sharp in/on the ground. In addition, if the field in which you pitch your tent gets muddy the footprint groundsheet will prevent the sewn-in groundsheet from getting dirty and therefore it is the only item that will need washing down when you get home or when you pack up.
Outwell footprint groundsheets are made using tough wearing polyethylene. They are waterproof and come complete with eyelets (or webbing tabs) so that they can be pegged out easily and a a small pack of standard tent pegs for the purpose.