Serverless is a catch-all term for apps and products that don’t require you to manage your own servers.
Every app runs on a set of servers somewhere, and there’s high overhead involved in managing them. Serverless just means that your app – or the infrastructure you pay for – doesn’t require server management. In practice, people use the term to refer to a function-based architecture instead of an app-based one. Each major cloud has a serverless platform, the most notable being AWS’s Lambda.