Local allows servers to run in browser threads where they host HTML and act as proxies between the main thread and remote services. The server threads run in their own Web Worker namespaces and communicate via Local's implementation of HTTP over the postMessage API.
This architecture presents many interesting opportunities for new kinds of web apps. Want to learn more? Paul wrote an article outlining four potential use cases that Local enables. Give it a read, check out the docs (which are built using Local), or view the project's source code on GitHub.