DynamoDB is a NoSQL database service with nearly infinite scalability and flexibility.
NoSQL means that you don't have to specify a schema which declares what kind of data you store (like you would do when you use a relational database).
DynamoDB can be operated in different setups. You could for example use DynamoDB to store your data in key-value pairs. We'll use the document setup which means that we'll store JSON objects in our database.
DynamoDB is "just another AWS service" we'll use throughout this book to store data in a database. It's absolutely not a must to use DynamoDB so that the Serverless framework works.