When a web browser requests a web page or file from a website it sends the user agent as part of the request. The user agent contains information that identifies the web browser make and version e.g. Firefox v32 or Internet Explorer v11. This information is sometimes used by the website to send a tailored response that is appropriate to the capabilities of the web browser. Usually you wouldn't want to set the user agent yourself but the ability to specify the user agent can be useful to people who develop websites and need to check that they look ok on a variety of browsers.
