The freeholder of a property owns it outright, including the land it’s built on.
A leaseholder owns a property for a fixed period of time, after which the lease must be extended or ownership returns to the freeholder. Most flats and maisonettes are owned leasehold, which means that you own the property in the building, but not the building itself.
As a leaseholder, it is also possible to buy a share of the freehold.