To quote Mark Jaquith’s helpful post on this topic:
“If you update your WordPress theme’s style.css, you may have noticed that you have to “force-reload” your site in your browser to see the changes. This is because your browser keeps a copy of the CSS cached on your hard drive. Depending on how your server is set up, it may not check for a new version of the stylesheet for a couple hours, or longer! And even if you force-reload to see the changes, visitors who have previously accessed your site may still get the old CSS.”
That post is from 2009 and WordPress coding conventions have changed a bit since then, so I’ll provide an updated solution:
This tells WordPress to enqueue the stylesheet/js file just as it normally would, but to append “?ver=” followed by a Unix timestamp representing when the file was last modified to the end of the filename before doing so ( see the image below). This ensures that the version of the file enqueued is the most recent, so everyone instantly sees your changes.