When you purchase a leasehold property you agree to lease that property from the freeholder for a certain number of years. This term can be increased by extending your lease. A lease can be extended at any time, and a longer lease comes with many benefits.
When you buy a lease, it gives you the right of living there for a fixed number of years, but you will not be the responsible for things such as the common parts of the building, car park and any communal gardens. Once the lease runs out the ownership of your flat will return to the landlord unless either you buy the freehold of the building or the lease is extended.