systemd is like the service manager for your Centos and other modern Linux distributions (including Debian/Mint/Ubuntu) allows you to enable services, stop them, restart them, check their status and even reboot your system.
The key commands or arguments you will use with systemctl are the following:
list-units [PATTERN...] List loaded units
In Centos 7 the days of editing the "kernel" line and adding "single" are gone. On top of that sometimes after a new install passwords do not work (possibly because requirements were not met but the installer never mentioned this?).
1.) On bootup edit the bootline by entering GRUB.
Find the line that says "linux16 /vmlinuz"
Edit the part that says "ro" and change with &q........