Freehold and leasehold refer to different types of ownership on a property. As a general rule, people consider freehold to be superior as it has no restrictions. If you own a freehold property, you will own the building and also the land that it sits on. This ownership is forever and you will be able to pass the property on to other members of your family.
Leasehold ownership means that you own a property but you do not own the land that it sits on. The lease is also only for a certain period of time, often 99 years. When a lease begins to get shorter, the property loses value. If you try to sell a property that only has 10 years left on its lease, it will be much lower in value than a property with an 80 year lease. Flats are usually leasehold, meaning that someone else owns the land and the building.