The Keyboard Maestro Engine is a background only application that enables all of Keyboard Maestro’s features. It responds to your Hot Key presses, watches the time, tracks applications and maintains your clipboard history, handles remote web requests and receiving clipboards, and, of course, executes your Macro Actions. It should be running at all times, so we recommend you enable the “Launch Engine at Login” preference in the Keyboard Maestro General preference pane.
The Keyboard Maestro Engine is also launched whenever you launch the Keyboard Maestro editor, and then remains running until you logout unless you explicitly quit the Keyboard Maestro Engine.
While it is not required to have the Keyboard Maestro Engine launched at login, if it is not then your macros will not start working until you launch the Keyboard Maestro editor (which you normally only need to launch when you want to actually change your macros).
You can enable the “Launch Engine at Login” preference by selecting Keyboard Maestro ➤ Preferences, and turning on the “Launch Engine at Login” checkbox in the General preferences.