A more thorough explanation:

A writ of ad quod damnum is a legal document that directs the sheriff to inquire of jurors under oath to what damage a grant (such as a fair, market, liberty, or other franchise) would cause to various people if the king were to make the grant. The writ was issuable from the court of chancery.

For example, if the king wanted to grant a new market to a town, the writ of ad quod damnum would be used to determine if the market would harm any existing markets in nearby towns or cause any other damage to the community.

This writ was important in protecting the rights and interests of the people, as it ensured that any grants made by the king would not cause undue harm or damage.