The simple answer is a system with no servers to manage. The most accurate technical way to describe a serverless system is by the statement:

Serverless architecture (or system) is an event-driven ephemeral FaaS (Function-as-a-Service) which is a type of cloud-computing service that allows you to execute code in response to events without the complex infrastructure typically associated with building and launching microservices applications. This allows our development team to focus solely on individual functions in your application code.