It is your own password.
The first user you create in Ubuntu is added to the group named
admin. Users in this group can perform system tasks by providing their own passwords. So, let's say you have a family with a mother, a father and a child. That's three users. Parents are allowed to install new applications, but the child is not. Since father installs Ubuntu and is the first user, he is automatically allowed to do this. Then he adds mother to the
admin group and then she is also allowed to install apps. But they don't have to share a password. When they're asked to authenticate in order to perform system tasks, they provide their own passwords.
This mechanism is called
sudo and is very similar to Windows'
sudo has been used in Unix-like systems for a very long time and is probably the main inspiration for