Drupal Tutorial : Administration : Access Control
Using the drupal access control menu, you can assign/restrict privileges to visitors of your website. The access control list at (administer-> access control) is quite self-explanatory.
A typical use of the access control is when you want to make some parts of your site available only to registered users. The default access control rules are quite safe, but you might want to change them to satisfy better your needs. Spend some time playing with the access control menu, to better understand what you can do.