This is a place where you can save all the contacts you found in the Prospecting tab. Create Lists to keep all your contacts organized.
Leads appear here after you hit 'Open and save selected' button. You will be offered to save leads to:
Default list - a list where every lead you open is saved
New list - create for your particular searches and give a correspondent name, for example "Sales managers from Alabama".
Please note that you can not add more than 5000 people to one list. You can export the whole list to a CSV file anytime.