So, I'm familiar with how to build a website using HTML, CSS and JS. I'm also quite confident using ReactJS to build webApps and even fetch data from some APIs.

However, I've never used nodeJS or similar technologies to build myself some kind of Backend. What exactly do I need it for? And what kind of code does a Backend typically contain, as a lot of logic already works in the Frontend?

Is it just sort of an API that connects my frontend to some database?