It’s useful in case you forget your password and unable to access your system.

All you have to do is:

1. startup your machine and select the kernel that you use at the grub menu
2. press e to start editing
3. Find the line that starts with linux and change ro to rw and add init=/bin/bash then press ctrl+x
4. You will directly enter the console as root, just type passwd and write your new one

That’s it, Enjoy!

Source: https://help.ubuntu.com/community/LostPassword