When booting into single user mode, you may be required to enter the root password - not that useful if you don't know it.

Procedure to reset the root password

Start the system and on the GRUB 2 boot screen, press the e key for edit.
Remove the rhgb and quiet parameters from the end.

Add the following parameter at the end of the linux16 line, or linuxefi on UEFI systems:

init=/bin/sh

The Linux kernel will run the /bin/sh shell rather than the system init daemon. Therefore, some functions may be limited or missing.