node_modules directory, in which the dependencies of the project is being installed. This is a problem because it will cause Git to track dependencies, which can amount to Gigabytes of space, slowing down operations such as switching branches. You don't need to track someone else code.
The solution offered by Git is to use a special configuration file named .gitignore. You can put one in your project and list the name of files you want to ignore there.
$ echo "node_modules" > .gitignore
git status will show .gitignore as untracked. Add and commit in into your repo. Now you're good.