This is a condition found in the Title Deeds or lease for a property that a buyer must agree to if they go ahead and purchase the property. This is typically applied to the current property owners and anyone who may assume ownership in future. The word covenant means agreement, and it will be contained within the conditions of the deeds or lease.
A restrictive covenant highlights something that the owner cannot do under the terms of that covenant. For instance, a property may have a restrictive covenant that prevents the owner from using the property to run a business. If the land on which the property is built is owned by a farmer, that farmer could add a restrictive covenant that prevents the property owner from buying chickens or other animals (other than domestic animals). These are just a couple of examples; there are many others.
Conversely, a positive covenant could require the owner to do something. This might mean maintaining a fence around the perimeter (or a portion of the perimeter). If the property is in a conservation area or is of a certain age, a covenant could require the new owner to maintain existing windows rather than replacing them with modern uPVC ones.
If you are thinking of buying a property, you should always make sure your solicitor looks at the presence of any covenants attached to that property. This applies to both restrictive and positive covenants. While positive ones might sound like a good thing, they can still be restrictive. The cost of maintaining old windows (and the likelihood of higher heating bills thanks to their inefficiency) could be viewed as a negative, putting you off the purchase before progressing further.