A Linux operating system has a core program called a kernel, which has the job of creating and maintaining an environment where other programs can run. Other programs include the login prompt, the shell that lets you execute programs, etc. However, there is a chicken-and-the-egg problem: once the environment is running, how do you run the first program? This is solved by having the Linux kernel run a program called "init" as the first program, which manages everything else (bringing up services, executing the login prompt, etc). There are multiple versions of this "init" program with different features and idiosyncrasies -- the Debian committee voted to use one called systemd rather than some competitors. Switching versions of init requires system administrators to relearn how to do many things, so the committee spent a lot of time weighing a lot of factors before making this decision.