This guide is intended for the absolute beginner to AWS. It assumes you know little to nothing about AWS.
A "serverless" function ran without provisioning a certain server for a given amount of time. The concept of serverless is a huge topic, but the basic idea is that the starting and stopping of the server is abstracted away. When you ask it to run, it runs. After the code is done, the server stops running. You are only charged for the server when it's running.
They are completely stateless, and are essentially a new boot every time they run. This means that session level things maintained by a Lambda, such as auth session, are not possible.