su is known as either switch user or super user. The command by itself will prompt you for the root password to become root(super user). The command su followed by another user name will prompt you for that users password.
If you want to get any help you need to provide us with specific technical details of what is happening. "It didn't work" simply is not enough information. Post any and all errors that you recieve on the command line or else we are just taking stabs in the dark about why "It didn't work".