A "changelog" is a brief record that tells you about the changes and updates made to something, like software, a website, or a project. It helps people know what has been added, fixed, or modified. Full definition
Samsung itself hasn't listed the changes introduced with Experience 9.0, but SamMobile has provided a full changelog of new features. (pocket-lint.com)
You can find the official changelog of Update 3 with the changes here. (windowsphonearea.com)