A leasehold property means that you have a lease from the freeholder (sometimes called the landlord) to use your home for a number of years. Typically leasehold properties are flats, but it can apply to houses as well. The leases are usually long term, often 99 years or 125 years and as high as 999 years, but can be short, such as 40 years. Within your lease you will find various regulations that must be adhered to by all parties.
This type of long leasehold property is not to be confused with a Assured Shorthold Tenancy, where a lease is granted to a Tenant from a Landlord for typically 6-12 months. In this scenario, the Landlord may also be a leaseholder.