Simple definition
systemd-logind manages Linux user logins, sessions, seats, and related access decisions.
Purpose and use
systemd-logind manages user sessions, seats and selected power-management or logout operations.
How does it work?
It tracks logged-in users and exposes interfaces that let system components coordinate session state.
Example and verification
Investigating sleep or logout problems requires distinguishing sessions, inhibitors and applications still running.
Points to watch
logind does not replace authentication mechanisms or each application’s authorization rules.
In short
It coordinates session lifecycle rather than deciding every user permission.