What is server-less architecture?

Google tells me this:

Serverless architectures refer to applications that significantly depend on third-party services (knows as Backend as a Service or "BaaS") or on custom code that's run in ephemeral containers (Function as a Service or "FaaS"), the best known vendor host of which currently is AWS Lambda.

I can grasp the meaning until the BaaS part, but from there everything goes above my head. Can anyone explain it to me like I'm 5? :D