egg-ts-helper is a development tool that generates TypeScript definition files (d.ts) for Egg applications, enabling IntelliSense for controllers, services, proxies, and other Egg components. It automatically creates type definitions for files in directories like app/model, app/controller, and app/service, and supports custom loaders and generators. The tool is primarily used by Egg.js developers working in TypeScript to improve development experience through enhanced type checking and autocompletion.