Grandfather clause is a contractual or statutory provision exempting persons or other entities already engaged in an activity from rules or legislation affecting that activity. It is an exemption based on circumstances existing prior to the adoption of some policy and was used to enfranchise illiterate whites in US south after the Civil War.
A grandfather clause allows the current status of something pre-existing to remain unchanged, despite a change in policy which applies in the future. For example, the 22nd Amendment to the Constitution specified term limits for future presidents, but did not apply to the president (Truman) in office when Congress passed it. In another example, zoning regulations may disallow property to be used for certain purposes, but contain a grandfather clause allowing properties with such uses already in existence to legally continue. Whether a pre-existing use will be grandfathered into a new zoning regulation disallowing such uses depends on a variety of factors and political influences, so there is no definitive rule on whether a particular use will be grandfathered into the new regulation.
In another example, businesses serving the public are required by the Americans with Disabilities Act (ADA) to make their premises handicap accessible. If you own, operate, lease, or lease to a business that serves the public, then, you are covered by the ADA and have obligations for existing facilities as well as for compliance when a facility is altered or a new facility is constructed. Existing facilities are not exempted by "grandfather provisions" that are often used by building code officials.