Before version 8, there were a couple ways of triggering Keyboard Maestro macros remotely, but none of them were particularly great.
You could use the Keyboard Maestro web server, either directly or from the Keyboard Maestro Control iOS application (which is very old and outdated, sorry!), but that generally only worked on a local network (unless you have a real NAT-free connection to the Internet or set up port forwarding on your router, both of which are very rare these days).
Or you could make some sort of hack using Dropbox or iCloud Drive and sending a file to your Mac somehow to trigger a macro that way.
But Keyboard Maestro 8 introduces the Remote trigger that lets you trigger a macro from anywhere on the Internet. This allows you to integrate with network services like IFTTT or HealthChecks.io. For example, I use HealthChecks.io to monitor my servers and report if they go down, and it reports by triggering a macro that puts a big red floating window on my screen as well as sending me an iMessage.
The Remote Trigger has essentially two parameters, which are usually two unique IDs (essentially passwords). One can be set to an ID for your Mac (so that if you sync your macros, only one Mac will fire), or it can be set explicitly (in which case it would be synced to your other Macs and all of them would fire). The second one is always explicit.
While you can change the unique IDs that are pre-selected, I would encourage you to leave them as the random IDs that Keyboard Maestro generates unless there is a very good reason to change them. A reason you might change the IDs might be for a trigger you explicitly make public.
Note that these two IDs together are shared by everyone using Keyboard Maestro. So if you make a Remote trigger for "a/b", don't be surprised if others end up triggering - keeping the random UUIDs is still a good idea for this reason.
There are a bunch of caveats for the trigger, not least among them are:
Anyone who knows the IDs used in the Remote Trigger can trigger the macro - so keep them secure, and limit your use to macros that are relatively safe to run (for example, if someone managed to trigger my HealthChecks.io warning, no real harm would be done).
The trigger happens by connecting to a Stairways web server, and by Keyboard Maestro keeping in touch with the Stairways web server. That means that a) any Internet outage between the client, the Stairways web server, and the target Mac might block the trigger; and b) since Keyboard Maestro is not sold as a service, this part of Keyboard Maestro is not guaranteed indefinitely, eventually the server will stop working and so will this trigger.
I look forward to seeing what people do with this new facility.