This plugin help serve static content off CDN or some Offsite URL. You need to setup a "Pull Zone" with Origin point to your Site from your favorite CDN Provider: MaxCDN, StackPath, KeyCdn, BulugaCdn, BunnyCdn, etc...
Use CDN plugin replaces your local Url with CDN Url for non-admin user (no edit toolbar). This mean that you have to use a different browser, incognito mode, or logout to see the result. This also help with debug and is safer to use because it does not break admin dashboard.
After add-on/package install, you can update settings @ Dashboard -> System & Settings -> Optimization -> CDN Settings