npm is a package manager for the JavaScript programming language maintained by npm, Inc., a subsidiary of GitHub. npm is the default package manager for the JavaScript runtime environment Node.js and is included as a recommended feature in the Node.js installer.
You can use it to install packages by rinning npm install <package name>. This will add the package to your package.json and it will add its files to the node_modules folder.
To install all packages that a package.json has included, run npm i. This is needed because the node_modules folder, the folder that actually has the packages, is not committed to projects (or at least it should not be).