A system that allows applications to communicate by sending messages to a waiting line. Like a post office where letters wait to be picked up.
When you upload a photo, it goes into a message queue to be processed for different sizes (thumbnail, medium, large) without making you wait.
All provide managed messaging to decouple services. SQS and OCI Queue are queue-centric services; Azure Service Bus Queues add enterprise features (sessions, transactions, dead-lettering); Google Cloud Pub/Sub is primarily pub/sub but can be used like a queue via subscriptions and pull delivery.