Billable items are different values that are due to be invoiced to clients. They mainly apply to Cost Plus and Time & Material jobs, and are created whenever new costs are added to the job.
In the example below, an expense is added to a T&M service ticket:
Then when this ticket is completed, the expense will be invoiceable to the client:
A list of all pending billable items can be found in the Manage Billable Items tab of your Invoices module.