The CONTACTS tab is where you list the individuals at this customer or supplier who you interact with.  You can add new contacts by clicking the + button to add a new row, or delete contacts with the - button. For each contact you can also specify their:

1. Name
2. Phone number and email
3. What kind of contact this is (see list below)
4. Enter notes or remarks about this individual
5. Click the pencil to edit contact information including multiple phones/emails in a popup.