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 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/7.54.1
for curl version 7.54.1.
You can set any value you like, using the option -A or --user-agent plus
the string to use or, as it is 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