A changelog is a file (or document) in which product owners record notable changes made to their product. The term "changelog" originally comes from software development and is derived from "change log", but it's been applied to other kinds of projects, as well. For example, Wikipedia has a page that lists notable changes made each day to its content.
There are many reasons why you might want to include one with your main project:
Product managers, business owners, product owners, or anyone that wants to communicate product improvements to users and stakeholders need to have a changelog.