You had an email send and are now seeing in an Email Performance Report that there are Pending Emails, a few hours after initiating the program/campaign.
There is a difference between Sending and Delivering. Marketo "sends" emails. More often than not, Marketo sends the mail relatively quickly. "Deliveries" are something that is returned to Marketo by the recipient email servers.
"Pending" generally means that Marketo is attempting to deliver a message. Marketo will try to send a message, but can receive a temporary bounce (essentially "Thanks, try again later"). When that happens, the mail status is set to pending while we retry. We will try to send the message several times over the next 24 hours (36 to AOL) before we accept that the message will not be delivered and change the status to "bounced".