Explain the concept of middleware in Koa.js.

Middleware in Koa.js plays a crucial role in handling various aspects of the request-response lifecycle. Middleware functions are functions that have access to the Koa.js context (ctx) object, which encapsulates the request and response.

These functions can perform operations on the context object, modify the request or response, or control the flow of the application. Middleware functions in Koa.js are executed in the order they are defined.