What are `.d.ts` files in TypeScript and why are they needed?

Hi everyone! I just started learning TypeScript and came across files with the `.d.ts` extension. Honestly, I'm a bit confused. The official documentation says these are "declaration files," but I don't fully understand their purpose. I have a project where I use the `lodash` library. I installed it via npm, but TypeScript complains that it can't find types for `lodash`. I had to separately install `@types/lodash`, and after that, `.d.ts` files appeared in the project. I tried writing my own `.d.ts` file for a simple function: ```typescript // utils.d.ts declare function greet(name: string): string; ``` But when I tried to use this function in my code, TypeScript said the function doesn't exist. What am I doing wrong? Please explain.