A safe senders list is a list of approved senders maintained by email recipients. A user adds a sender address to his safe senders list to ensure good reception as emails from the sender will not be subjected to additional filtering in the future. It is a whitelist system managed at individual level.
Email marketers encourage recipients in email and while subscribing process for being added to their senders lists to prevent email filtering and ensure deliverability.
The safe senders list term is in particular used by Outlook and Hotmail but it is also used by extension for all approved senders systems.
Strictly speaking a safe senders list differs from address books or contact lists. But by default, email addresses added to address books are commonly considered safe and will not be re-routed to a junk email folder.
Clickz is seeking to become a safe sender by offering a tutorial to recipients: