The word from the Latin ‘let him beware’ is a warning procedure in the context of probate. It allows a person to delay probate being issued until the caveat has been removed.
An example: if you are concerned about a recently deceased relative – you may suspect that his or her will was the result of undue influence – you can lodge a caveat at the probate registry which will prevent probate being issued until the caveat has been removed. Here is a link to the government's website explaining the process of rules on lodging a caveat.
The lodging of caveats is not always justified and executors are sometimes surprised to find that what they thought was an uncontroversial probate cannot proceed because someone has lodged an ill considered caveat. At this point it will almost certainly be necessary for the executors or the administrators to take legal advice from solicitors who provide a comprehensive probate service.