Enqueue:
Sending data from another system to Dynamics 365 F&O. When data needs to be transferred to Dynamics 365 F&O, it's placed into a queue (enqueued), awaiting processing within the Dynamics system.
Dequeue:
Sending data from Dynamics 365 F&O to another system. After the data has been processed within Dynamics 365 F&O, it can be removed from the queue (dequeued) and sent to another system as needed.
So in summary:
Enqueue is related to receiving data into Dynamics 365 F&O.
Dequeue is related to sending data from Dynamics 365 F&O to another system.