Phoenix is a web framework for the Elixir programming language, a set of libraries and conventions for building web applications, HTML pages and JSON or GraphQL APIs. It runs on the Erlang VM, the runtime that Elixir uses, which the site says can handle millions of WebSocket connections.
A developer creates a project with mix phx.new. Phoenix includes Channels and PubSub for real-time messages across one or more servers, Presence to know who is connected, generated authentication, a live metrics dashboard, and Ecto for Postgres, MySQL, SQLite and SQL Server. Pages use HEEx, templates of HTML with embedded Elixir and reusable components. LiveView, included by default, keeps each page as a process on the server: the first view is ordinary server-rendered HTML, then a persistent connection carries browser events to the server, and the server re-renders only the changed parts and pushes them to the browser.
Phoenix is free and open source under the MIT license; its development is sponsored by Fly.io and Dashbit.