When you buy a freehold property, you own the building and the land it’s on until you decide to sell it (though of course your mortgage company could still repossess it if you don’t keep up repayments). This kind of total ownership is known as ‘title absolute’.
If you buy a leasehold property, you only own the building (not the land it’s on) and only for a specified number of years. When the term of the lease expires, the property will belong to the landowner unless you can extend the lease. Bluntly, this means that however much money you paid for the property, you would lose it all if the lease were to run out. Although in practice this doesn’t often happen, it’s a risk that hangs over all owners of leasehold properties.