A grocery store sells a bunch of different types of items. Vegetables, Fruits, Meat, Cookies, Milk, Eggs, Sugar, Chocolate. They get these items from different companies around the world and make them accessible to those who visit the store.
Installing Node.js also installs NPM just double check by running
So back to the grocery store example. Lets say you buy some groceries. You buy Salmon, Pepper, Syrup and Pecans. You can be a savage and eat these items individually but chances are that if you combine all of these items you will get a dish that tastes much better.
It would be advantageous for you to have a package.json that you can use throughout your projects like as a blueprint. If you have a package.json with a list of packages you can run npm install and it will download all of those packages to your local project.
You can also see all of your libraries that need be to be updated by running this command
Now that you have a basic understanding of NPM go out there and make an awesome web app that utilizes NPM. It will make your experience as a developer much more pleasant.