Why are my subscribers on the pending list?
Contacts who subscribe to a list with double opt-in enabled will be marked with the pending status. It means that they received an extra confirmation email to verify their email address but they have not confirmed their subscription yet. When they open the email and click on the button there to confirm their email address, they will be automatically moved to the subscribed contacts.
Here you will read more about the double opt-in.
You can find your pending contacts by changing the displayed contacts within your list.
If you are sure that your pending contacts want to receive messages from you, but for some reason, they cannot find the confirmation email to confirm their subscription, you can manually change their status to subscribed.
To do it, click on the contact and edit their status.