It's a string that your browser sends to websites with information about the browser itself and your OS. Sometimes that info will be used to block functionality.
Years ago I tried to use TurboTax from Firefox on Ubuntu. It wouldn't work because only Internet Explorer on Windows was supported. I changed the user agent to make it appear as though I was using a supported setup, and it worked flawlessly.
I haven't actually needed to use one in a long time, but an extension search for "user agent switcher" should turn up something that can do it.