The OS allows many processes to run on a computer at once. To allow processes fair time at using the scheduler, the scheduler gives them all a timeslice and calls this an epoch.
When booting the OS, after the kernel has set up its code, the root
user starts its init system (generally systemd) which can start
arbitrary programs as arbitrary users (it forks from the root user, and
then uses setuid() to allow it to become a different user
process).
You can fork() and execve() to run
different processes on a computer in Linux.