A more thorough explanation:

Revocation of a will by instrument means that a will is no longer valid because a new will has been executed that contains a clause specifically revoking the former will.

For example, if John creates a will in 2010 and then creates a new will in 2015 that includes a clause stating "I hereby revoke all prior wills and codicils," the 2010 will is no longer valid.

This type of revocation is common when a person wants to update their will to reflect changes in their life, such as the birth of a child or a change in marital status. By executing a new will that revokes the old one, the person ensures that their wishes are up-to-date and legally binding.