Hm.. A little hard to explain. Imagine, you are surfing a joomla web-site. You open a page with some content and feel a passionate desire to create an account. You start the process by clicking a login link, fill in forms and so on. Finally you perform the first login - and see your profile page instead of the article you have been viewing before you start the sign up process.
This plugin redirect you to the last page, you has been viewing before starting to sign up.
There is also a possibility to perform after login when activatin a user - when clicked by the email activation link.