What does amendment thereto mean?

It just means an addition. For example, say you draft a will and you decide leave $5,000 each to your three kids. You then you get a car as a gift after the will was finished and decide to leave it to one of the kids. Instead of re-drafting the will, you would ammend it. The additional section would likely begin by referring to the original will and specifying that this is an "ammendment thereto". Essentially, nothing written before is changing, you are just adding something new. Hope this helps.