The init system is the first program to start when a computer boots. Its role is to start other program automatically and to let you manage them more easily.
How to know which init system I have?
You can use this little script.
if [[ `systemctl` =~ -\.mount ]]; then echo "SystemD"; elif [[ -f /etc/init.d/cron && ! -h /etc/init.d/cron ]]; then echo using "SysVInit"; else echo "Can't tell"; fi