The administrator password is created when a VPS account is configured. It allows accessing all the functionality of the server. When you install the operating system on a VPS, the person with administrator-level access gets a password. This is called a root password in Linux/Unix systems. In Windows, it is more commonly referred to as an administrator password. The original password should be changed as soon as possible. This is for security reasons. In the most recent Windows versions, a primary account is also an admin account by default. This is not the case in older versions like Windows XP. In those operating systems administrator password should be used to access the Recovery Console. It is also needed if you want to restart the server in Safe Mode.
The new password shall be stored in a safe repository. You should never forget this password. To prevent such a situation, you can use a password manager. This program functions as a password vault. All your important passwords, including your administrator password, can be safely kept in there.
Windows has taken provisions too. The password reset disk is their solution. This disk will allow you to access your administrator account at any time. It will work even if you have changed your admin password many times.