The User-Agent is a header that each client can set in the request to inform the server which user-agent it is. Sometimes servers will look at this header and determine how to act based on its contents.
The default header value is 'curl/[version]', as in
User-Agent: curl/7.54.1for curl version 7.54.1.
You can set any value you like, using the option
--user-agentplus the string to use or, as it's just a header,
-H "User-Agent: foobar/2000".
As comparison, a test version of Firefox on a Linux machine once sent this User-Agent header:
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:58.0) Gecko/20100101 Firefox/58.0