What is an arbitrary URL?

I have created a plugin for wordpress but I don’t understand what an “arbitrary URL” is. Here’s what I’ve found:

When installed, the plugin has to make a custom endpoint available on
the site of WordPress. By “custom endpoint” we refer to an arbitrary
URL not recognized by WP as a Standard URL, like a permalink or so.
Please note that this is not a point of REST connection. When a
visitor navigates to that endpoint, the plugin has to send a request
HTTP to a REST API endpoint. The API is available at https: //
jsonplaceholder. typicode.com/ and the connection point you are going
to to call is / users. The plugin will parse the JSON response and use
it to create and display a table HTML.

I would like you to tell me what this arbitrary URL is and clarify the question in general please.