Nothing. Systemd is a suite of software that handle a lot of the low-level operations on Linux (In particular, the service manager, some network configuration, along with some other stuff). Historically, those operations were handled by different services (like SysvInit).

A lot of people are mad about it for a lot of reasons, but if you're not a system administrator, it's probably better to stick with systemd, since it's what most of the linux community has standardized on, and thus it's much easier to find information online about using those systems.