From the docs in case you haven’t seen: https://docs.npmjs.com/getting-started/what-is-npm
Are you familiar with the concept of package managers? https://en.wikipedia.org/wiki/Package_manager
NPM is a package manager that enables you to use packages (software) written by others that your application might need. This is what the term ‘dependencies’ refers to.
It’s normal for a project to have many dependencies, so rather than manage each of these individually a package manager allows easier management (installing / updating / removing).
You might also have seen the reference to a ‘package.json’ file. This documents each package and which version your project depends on, this makes it easy for others to mirror your project by setting up from the package.json.
Hopefully that helps?