npm
npm is a tool that downloads ready-made pieces of code, so you do not write everything yourself.
In a simple example
You type npm install and it fetches all the pieces your project needs.
Why it matters to you
Most modern sites are made from many such pieces, so you will see this command in nearly every guide.
Do not confuse with
npm is not a website you visit. It is a command you type, and a store of code pieces behind it.