Systemd is an implementation of an init system, with some extras too.
Init systems are traditionally the first thing that is executed when the kernel boots and passes control to userspace. They're responsible for starting all the services that are configured/enabled.
In addition to this "basic" functionality systemd has extras which allow it to handle dependencies, start services on-demand, and handle logging, etc.
Beyond that you'd be best reading the documentation on the systemd site - I've never used it, but the basics don't seem too hard to understand.
