Each of your contacts is assigned a "health" rating (0-100%). This represents how complete the information about this contact is.

You can tell the completeness rating of a contact by looking at the blue health meter around your contact's avatar:

 

If you just have a name and a phone number the rating will be low. If you've added industry, company, location and full contact details, the rating will be high. 

You can then organise your address book by improving the contacts with the lowest ratings. You can see these by filtering by "health" in My Contacts:

Your entire contact list is also assigned an overall health rating representing the average rating of all your contacts. You can look at your address book health by accessing your address book diagnostics:

Here you can see the percentage completion for each field of your contacts.