A more thorough explanation:

A testimonium clause is a provision found at the end of a legal document, such as a will, that states the date the document was signed, who signed it, and in what capacity. It usually starts with the phrase "In witness whereof."

For example, a will might end with a testimonium clause that reads:

In witness whereof, I have hereunto set my hand and seal this 1st day of January, 2022, as my last will and testament, in the presence of the undersigned witnesses, who subscribe their names hereto at my request, in my presence, and in the presence of each other.

This clause serves as evidence that the document was properly executed and witnessed, and can be used to prove the validity of the document in court.