You subscribe one or more SQS queues to an SNS topic

when you publish a message to a Topic, SNS sends it to every subscribed Queue, fanning out the notification to those queues.

If SQS is highly available and scalable you might wonder why you would need fan-out. Reason is that you might want different things to be happening in reaction of an event, and you want to decouple this business logic.

Without fan-out your Lambda function would then be in charge of all the different responsibilities. No really decoupled, right?

You can subscribe multiple queues to the same topic to replicate a message over multiple queues.

Each queue will receive the identical message and react accordingly.

There is an interesting tutorial here

On top of this you can also play around with filters and message attribute to replicate messages only to some queues. By specifying a Filter Policy in the Queue Subscription, you will then have your Publisher sending out the event, and SNS handle the logic to forward it to the multiple interested queues, ignoring those where the attributes don't match the business logic. (nice article here)