Some actions, such as updating the package lists, can only be performed as root. If you are not root and you try to update the package lists, aptitude will ask if you want to become root:

[D]

If you select “Become root”, aptitude will prompt you for root's password; when you have correctly entered it, aptitude will perform the action that required root privileges. You will still be root after the action completes.

You can switch to the root account at any time using the command Åtgärder → Bli root. Any changes you have made to package states will be preserved (but will not be saved until you quit aptitude).