Short answer
Restrictive covenants are legal restrictions that may limit how property or land can be used. Where recorded, they may appear in the Title Register or in filed documents referred to by the register.
Official source: https://www.gov.uk/search-property-information-land-registry
What covenants can relate to
Covenants can relate to building works, use of land, access, maintenance, alterations, trade use, shared areas or obligations affecting the property. The exact wording matters.
Which document should you order?
Start with the Title Register. It can refer to restrictive covenants, notices, rights and other entries. Some detail may sit in a separate filed document where available.
Official source: https://www.gov.uk/get-information-about-property-and-land/search-the-register
Scotland note
Registers of Scotland explains that Scottish title sheets can include burdens or responsibilities, which may include obligations affecting the property owner.
Independent service note
LandRegistries.com can help order title documents, but legal interpretation of covenants should be checked with a solicitor or conveyancer.