21st.dev VSCode Extension is a browser toolbar that connects frontend UI elements to AI agents in code editors. It allows developers to select DOM elements, add comments, and let AI agents automatically generate or modify code. The extension supports multiple frameworks (React, Vue, Svelte, Next.js) and works with various AI agents including Cursor, GitHub Copilot, and Windsurf. Installation involves adding the VSCode extension and integrating a toolbar component into the development environment.