If you live in a leasehold property, in the simplest terms, you own the lease of the property but not the land it sits on.
When the landlord first sells a flat to someone, they do so by creating a lease for a specified period, so for example this might be a lease of 99 years from 1st January 1970. When the first owner sells the flat, they sell the remaining term of the lease.
The lease specifies how long you have the right to live in that property as a tenant of the landlord or freeholder (though you still have the right to sell the property.)
In the example above therefore, if the first owner of the flat sold it to someone else in 1990, the new owner would have 79 years remaining on the lease. Someone buying the flat in 2019 (if the lease has not been extended) would have 50 years remaining on the lease.