Running a program and a dev server
Building software means running it. Most of the time you'll type the name of a program followed by what you want it to do.
node app.js # run a JavaScript file with Node
python script.py # run a Python file
npm run dev # start the project's development server
That last one is special. A dev server is a program that runs your app on your own computer so you can see it in a browser at an address like http://localhost:3000. The key thing to understand: it keeps running. The will look "stuck" — it stops giving you a fresh and instead shows a stream of activity. That's not frozen; that's working. Your app is alive for as long as that command runs. Open your browser, visit the address it prints, and you'll see your app.
When you're done, you stop it (next section).