Explain 'Ambient Modules' and '.d.ts' files.

Ambient modules are declarations that describe the shape of existing JavaScript code without providing an implementation.

Declaration files (.d.ts) contain only type information. They are used to tell TypeScript how to interact with pure JavaScript libraries. When you use declare module 'pkg-name', you are creating an ambient declaration. This allows you to use a library as if it were written in TypeScript, providing autocompletion and type checking for third-party code.