You need root's password for su.
Ubuntu sets it to random characters to prevent you from logging in as root. This is because some users might get annoyed with having to type a password every time they do administration tasks, and decide to log into their desktop as root, which is a VERY bad idea.
If you want to be able to use su or log in as root (again, logging into your desktop as root is a bad idea and never necessary), then there is a way to set root's password to one you know. Enter this command, and follow the prompts:
Now you can use su.