The Remote Trigger action (v8+) triggers a remote macro that has the Remote trigger. It does this by connecting to the remote Stairways Software web server using the appropriate URL for the matching Remote trigger.
It is basically just a simplified version of the Get a URL action.
The result from the server is the number of listening macros that the server currently knows about. So if it is 0, then no macro will be triggered. If it is more than 0, then probably that many remote macros will be triggered (though the normal caveats apply about anything that happens over the Internet).
Enter the information from the matching Remote trigger.
You can (v9.0+) include a parameter if desired.