Simply put, it refers in this context to preserving what is presented and no more.

It might be an important feature, a fireplace or the whole property that requires this approach, the concept is the same-minimal intervention- that is doing only that which will preserve the material, aesthetic and function of the feature, only taking action if by not doing so the feature will suffer significant damage or loss. Any intervention must be in keeping with the structure, but should also be recognisable as an intervention too, so that it is not confused in later times with the original- honest and yet distinct, in keeping with the aesthetic- buying it some time.