This refers to the tenure of the land that the property is being built on. Thus, home owners who bought a 999 years leasehold home will own it for 999 years, after which the land will be revert back to the land owner at the end of the lease.
However, a period of 999-years is a long time. The home can be lived in for many generations. Thus, a home with 999-year leasehold is practically similar to a freehold property.
Many home seekers who intends to own the home for many years or for legacy planning, looks to freehold or 999-years leasehold properties for their purchase.