The bottom portion of the Contact Screen will show Contact Notes, Custom Fields, and a map view for the contact's address.  You may toggle between the three views with the blue/gray tabs in the top right.  

Notes:  Will show all notes that have been entered for the contact.  This includes notes that other users have left, and are separate from disposition notes.  To create a new note click Add Note. 

Custom Fields:  Are entirely controlled by your admin and may or may not be set up in your Sales Sling.  They are there to provide fields for data that might not have a place to live in the Contact Information section.  For example, if you are collecting a customer's credit rating, there is no field for that in Contact Info, so your admin may create a custom field to place that data in.  Speak with your admin about what, if any, custom fields they would like you to use.  

Map:  This tab will pull up a Google Map view for your contact's address.  If available.