A facade easement is a legal agreement related to historic buildings that need to be preserved. It is a type of easement where the owner of a building with a historic facade is legally obliged to preserve the facade's integrity and looks, and cannot make any modifications to it which would take away from the facade's historic value. Facade easements are traditionally handled by various organizations dedicated to the purpose of preservation of historic sites and buildings, and they're usually the entity that oversees the proper execution of the easement.
A common misconception is that a building under a facade easement is completely precluded from any modifications - this is rarely the case, as the owner of the building is still given the right to make various alterations to the building's facade, when they do not end up devaluing its historic properties. For example, building attachments to the building which do not cover or obscure the facade in any way, or even removing parts of the building as long as the facade and its integrity are preserved completely.
A facade easement is beneficial to the owner of the building, as it's eligible for tax deductions - and that's the most common reason for owners to agree to facade easements, despite this interfering with their regular business plans. Tax deductions aren't universally applicable to facade easements though, and in some cases the easement may be imposed by the government when the owner is reluctant to agreeing to one.