Simple definition
A systemd service unit defines how a process starts, stops, restarts, and depends on other units.
Purpose and use
A service unit describes how systemd starts and supervises a program or task.
How does it work?
Its type, dependencies and completion rules determine how state is interpreted.
Example and verification
For startup failure, correlate configuration, exit status and the unit’s journal.
Points to watch
Enabling startup does not mean the service is currently running. A completed one-shot task may also be inactive without failure.
In short
Current state, enablement and execution outcome are different pieces of information.