“Pending” means that the emails are scheduled to be sent automatically according to the campaign timing.
When you first set up your campaigns and activate them, you will see a number of emails “pending” for each campaign on the Campaigns page. They will be sent automatically when the scheduled time comes; no further action is needed.
You can see which orders are scheduled to receive which campaign by going to the Scheduling page and filtering by “Pending” status.
Emails may also be "pending" due to the Daily Email Limit.