When sending a 1-to-1 message, goTenna Mesh will try to determine whether the intended recipient received it. If goTenna Mesh receives an acknowledgment of receipt at the other end, you'll see a delivery confirmation check mark in the chat window. If the message wasn't delivered successfully OR if the acknowledgment receipt is not received, you'll see “Delivery Unconfirmed”.
You can manually try to send the message again (usually moving to higher ground or just trying from a different location can help a lot).
(Note: Delivery receipts are only available in 1-to-1 chats in the standard app.)