Deferrals occur when we are ready to send an email, but the destination mailserver effectively says “not now, try again later”. Sends get deferred regularly and this is usually nothing to be concerned about for good reputation senders with strong contact engagement.
Email providers defer emails in order to sample how batches of contacts interact. If there are many opens and clicks, along with low bounces and spam complaints, they will accept another batch. This is repeated, often in growing batch sizes, until, with a “good” send, they will just accept the whole lot as fast as we can send it.