A caveat is a legal notice that is filed, which states that a certain action or actions may not be taken without the person who issued the caveat being informed. This is often used in probate law when there is a beneficiary contesting a will.
A caveat creates an additional step that must be taken during the course of doing something else. An example is when a property is sold and the money is put into a trust for release to benefactors when a person dies intestate; one of the benefactors might put a caveat on the sale of the property so that they are informed ahead of the sale and have a chance to confirm that they agree with the sale price.