If it's the old type box you get to keep it and can continue to watch all freeview channels but not record any or watch the ones you already have recorded. If it is the new Sky Q box then these have to go back to them.
^^^ I concur with Theshedman's post, except that the list of channels you'll be able to receive (if you're not required to return the box) is actually more extensive than those on Freeview. The full list is here:
https://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E