What does the npm -S flag mean?

-S is shorthand for --save, and it adds the package you're installing to the dependencies in your package.json file (which can be created with npm init). However, --save or -S is totally unnecessary if you're using npm 5 or above since it's done by default.

The 'S' option is the Save option in npm. It adds the npm package to your dependencies for your project. It's the same as --save.