Section 5 Notices must be served by any freeholder looking to sell their freehold to an unrelated third party.
This is often known as the 'right of first refusal'. Leaseholders of the relevant building(s) must be notified of this intention and be given a chance to group together and make a rival bid either at auction or directly. If the freeholder fails to do this before selling the freehold, they could forfeit it to the group of leaseholders.
It is important to act very quickly if you receive a Section 5 Notice, as there are very tight timescales to consider.