In this article from TechRepublic Premium you will learn how to manage systemd on your Linux systems.

From the article:

systemd is the initialization system for Linux that replaced the aging collection of startup scripts, often called System V. Those individual scripts were responsible for starting various functions for the system and were controlled by a “superscript.” One of the biggest problems with this init system is that if the init daemon couldn’t start, none of the necessary processes could then be initialized, which stopped a system at the kernel panic stage.

In other words, the machine couldn’t complete the boot process.

Over time, it became imperative that System V be replaced. What wound up becoming the de facto standard, brought with it much controversy. Some users found systemd to be too inflexible, and monolithic in nature.