If you see an applicant in the Pending Inbox with the status of "Unpaid" it means they have completed their application and clicked submit, but not taken the final step of entering their payment details and processing their payment.
If you want to communication with an unpaid applicant you can manually import them to the Inbox, where you can send them an email or sms. Once they are imported to the Inbox you can also view the information they have entered.
See "How do I import Pending Applications?" for detailed steps.
A related FAQ is "What do the Statuses on the Pending Inbox mean?".