node.js provides a runtime environment for building server-side applications. It is written in Javascript, and can be run on any platform within the Node.js runtime.
Heroku is a cloud-based platform that allows you to deploy a node.js server to the web.
When Heroku received the source code, it fetched all the dependencies in the package.json, creating a deployable slug. The platform then spins up a dyno, a lightweight container that provides an isolated environment in which the slug can be mounted and executed.