The request user-agent header is passed by the browser to the server to let the server know what type of browser the user is using. The user-agent allows the server to customize the response returned if needed. For example, if the browser can handle html5, the a response with the video tag can be returned.
A computer using windows version 10 and Firefox as a web browser, the response is the following: user-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0
https://en.wikipedia.org/wiki/Internet_Protocol