A 404 is just someone/something requesting a resources (path, image, JS file, CSS file, etc.) that your web server and Craft cannot resolve on your site.
If this is on a publicly facing website, there's about a 99.99% chance it's just some script/bot probing different URLs for various endpoints to execute known exploits. It's extremely common on any public facing website.
If you look in your actual
craft/storage/runtime/logs files instead of the log viewer in the Control Panel, you'll see the same 404's being logged along with the
REQUEST_URI. That will show the non-existent path/resource that was requested.