The Public Web trigger makes a macro available without further authentication via the Keyboard Maestro web server. See Set Setup and Use of Web Server.
Any macro with the Public Web trigger can be triggered by anyone without the username or password, assuming the Keyboard Maestro web server is enabled, and assuming they can connect to your Mac (which is usually anyone on your local network, but depends entirely on how your network is configured).
Public Web Trigger on a Macro
To Execute a Specific Macro
https://localhost:<Port+1>/action.html?macro=<YourMacroUUID>&value=<TheTriggerValueToPass> or http://localhost:<Port>/action.html?macro=<YourMacroUUID>&value=<TheTriggerValueToPass> (by default, the <Port> is 4490, <Port+1> is 4491)
As with all triggers, macros are only available if they are currently active, that is if they are enabled and in a Macro Group that is enabled and active.
For example, if you are running some sort of process on your Mac that occasionally fails, you could write a script to restart it and make it available as a Public Web triggered Macro, which you (or anyone else) could then execute to restart the process.
For a complete example with a macro, see MACRO: @Web Open URL Passed from KM Public Web Server.