What is the main usage of index.d.ts in Typescript?

It's Similar to .h file in C Or you may imagine an interface subsystem in any other language like java or php

The file contains function prototype (declaration & typing) of an underlying library

for reference on how to generate a .d.ts file from a library.js reefer https://www.typescriptlang.org/docs/handbook/declaration-files/dts-from-js.html

*.d.ts files are used to provide typescript type information about a module that's written in JavaScript, for example, underscore / lodash / aws-sdk.

This will allow you to use the javascript modules without the need to convert them to ts without getting any type of error on your code.

for example, if you have a folder myAwesomeLib, with index.js and index.d.ts files

on your code, you will be able to import the code with

import { SomeMethod } from './myAwesomeLib';

or

import { SomeMethod } from './myAwesomeLib/index';

your typescript will rely on the .d.ts file to find the correct types for the SomeMethod

Edit: More about Declaration files https://basarat.gitbooks.io/typescript/docs/types/ambient/d.ts.html