The browser cache is a local storage area where your browser saves copies of files it has already downloaded: stylesheets, images, JavaScript files, and other assets. When you visit a page a second time, the browser serves the cached versions instead of re-downloading everything, which makes the page load faster.
This is the reason you can make a CSS change, save it, refresh the page, and see nothing change. Your browser is serving the cached version of the stylesheet, not the updated one. The fix is a hard refresh: Ctrl+Shift+R on Windows or Cmd+Shift+R on Mac. A hard refresh forces the browser to ignore its cached files and re-download everything fresh from the server.
If a hard refresh still doesn't show your change, you may also have a caching plugin on your site (common on WordPress) that's storing a cached version of the entire page. Clearing the plugin's cache separately is the next step. When Loupely Lens produces a diagnosis and you paste in the override prescription but don't see a change, clearing both the browser cache and any server-side cache is always the first troubleshooting step before assuming the fix didn't work.