Loosehead has correctly described leasehold.
However, with a shared freehold the freehold is in fact vested in a Limited Company specially set up for the purpose, and when you buy into a shared freehold this company issues a lease on your flat plus one share of the Limited Company. So assuming four flats in a block through the share you own one quarter of the entire freehold. The Limited Company is a management company for the block, so you have to make regular contributions to it for maintainance of the common parts and repairs. As you are a shareholder you have the right (duty) to attend the management company meetings and say how the block should be run. When your lease drops to 70 years or so the management company issues a new 99 year lease. The cost is not significant.