Webmachine is a toolkit for building RESTful HTTP resources in Erlang. It provides a framework for defining resources using URLs and handling HTTP requests through functions that specify allowed methods, request handling, content types, and more. The document demonstrates building a simple "hello world" resource with Webmachine and discusses REST principles like idempotency, encoding, routing, and using different HTTP methods like GET, PUT, DELETE, and POST appropriately. Code examples show defining a resource module and functions for request handling.