From the article:

workerd (pronounced: "worker-dee") is a JavaScript / Wasm server runtime based on the same code that powers Cloudflare Workers.

You might use it:

• As an application server, to self-host applications designed for Cloudflare Workers.

• As a development tool, to develop and test such code locally.

• As a programmable HTTP proxy (forward or reverse), to efficiently intercept, modify, and route network requests.