The 'Reset password' task is a simple one: it allows you to generate a new one-time login URL for a site.
The one-time login URL is bound to the account of the admin user (uid 1) of a site. It cannot be used to reset the password of another user on the site.
This can be handy in cases where the password for the admin user of a site has been forgotten. The 'Reset password' task is re-runnable and provides a convenient method of re-gaining access to a website from within Aegir.
In normal situations, viewing a site node shows a 'Go to' clickable link that takes the user to the site in question.
When the Reset password task has been run on a site, the 'Go to' link automatically gets changed to become a 'Login to' link that is the one-time login URL.
Once the one-time login URL has been clicked, this link returns to being a 'Go to' link.