As a dead letter queue, I can configure an SQS queue or an SNS topic with subscriptions. But your question is to understand the meaning of it.
To use layman’s terms, an event will not be successful every time it executes. When it fails, and retries and again fails, then a DLQ (Dead Letter Queue) is used to push these failed events to SQS queue or an SNS topic to check why the event failed.
For example, you are using a Lambda function, and at a point it starts failing again and again. Now these failed events will be sent to the configured queues or topics.
As you are new to AWS, I would suggest you to take a look at this informative AWS Tutorial and also if you are looking to become an expert in the same, check out the AWS Training page.