This can be a little confusing at first, so here is the explanation. If you purchase a fee simple property, you own the land and the improvements upon it. This is the standard way of owning a property. However, if you purchase a leasehold property, you are buying a lease for the land and, usually, you own the improvements. A residential leasehold might have been set for 55 years but if the ground lease is 30 years old, there would be 25 years left on the lease. Most banks require a minimum number of years remaining on the lease before they will agree to lend on the property. Some require that the lease be renegotiated with the ground lessor, often a family trust or large landholding estate. This can take more time than you expect and, in fact, lessors are not obliged to extend or renegotiate the lease.
With all this, why would anyone buy a leasehold property? Because it is often less expensive to purchase and the lease is depreciable, which may have some tax advantages for you.
As always, before purchasing any property, you should consult with your tax advisor and your attorney about how to protect your tax position and your estate.