The first process (PID 1) started by the kernel at boot; on modern distros, that's systemd.
When the kernel finishes its own startup, the very last thing it does is start exactly one userspace process with PID 1 — by convention called "init." Every other process on the system is a descendant of PID 1, directly or indirectly, and PID 1 has one job no other process has: reaping orphaned processes (children whose original parent died) so they don't become zombies forever.
For most of Linux's history, init meant SysV init — a simple sequential script runner using numbered "runlevels." Nearly every major distro has since replaced it with systemd, which does the same PID-1 job but adds parallel startup, dependency-based ordering, and built-in service supervision.
If PID 1 itself crashes, the kernel panics immediately — there's no fallback process to take over, which is why init implementations are written to be extremely conservative and hard to crash.
Under systemd, PID 1 doesn't just start things once at boot; it stays running the whole time as the service supervisor, restarting crashed services, tracking dependencies between them, and adopting orphaned processes system-wide.
kill -9 1 to "restart everything" — this either does nothing (kernel protects PID 1 from most signals) or panics the kernel; never intentionally target PID 1./etc/fstab entry or a broken systemd unit dependency is far more common.