User emailed an invoice to the client, but it did not get marked as Posted after emailing.
Marking the emailed invoices as Posted helps you to identify which invoices have been emailed out, as the invoices are marked with a blue line on the left in the list view and this prevents emailing them twice. Core marks the invoices as Posted only if you email them to the client or client contact without previewing them. That is, if you select the invoices in the Invoices screen and then click Actions > Email > Client or Client Contact without previewing them in the subsequent prompt. You can also mark the invoices as Posted manually using Actions > Mark as Posted.
Invoices also get marked as posted by other means e.g., if a payment is made on them. So this is not the best way to keep track of invoices that you have emailed out. You should rather use the Last Emailed On column in the Invoices screen to keep track of it. Check out this FAQ for details: How do I know if invoices have been emailed out.