When you pay a subcontractor, you can either write a check directly to the subcontractor, or enter a bill and then pay the subcontractor later when you're paying other bills.
For example, if the subcontractor charges an hourly rate, you would enter the number of hours. If the Qty column doesn't apply to this work, leave it blank.
Important: Entering the name here gives you the ability to invoice the customer for the work later. Also, you'll be able to create reports that show your costs and revenue from the work.