Newbie - What does it mean when you "modify a cookie" or when a user tampers it?

I'm trying to understand more about cookies, and I;ve heard these terms a lot. Unforutnatley, I' cant find any resources (probably searching th ewrong term). Could someone clarify/give an example of these 2 scenarios>

Submitted August 18, 2017 at 01:41PM by badboyzpwns