The AGPL applies copyleft to web services. If you're learning about licensing, it might be worth googling copyleft. Fascinating concept, and, in my opinion, something to subscribe to.

If you haven't already, check out https://choosealicense.com/licenses/ . This gives a broad overview of the common open source licenses. And if you're just starting out, one of the first things you'll want to learn is that the licenses fall into either a permissive or copyleft category. You'll want to make sure you understand the difference between those broad categories.

Shortly, permissive have less to no strings attached to use their code, and copyleft requires you to retain the same licensing terms meaning if you publish under GPLv3 then someone using/ modifying your code needs to also publish under GPLv3. Copyleft licenses ensure that open source code stays open source.

In very basic terms - GPL means that any modifications you make to a code base and distribute to public, you need to keep the license as GPL and open source all your modifications.

Once cloud started becoming a thing, the cloud vendors went "Well ackchyually🤓, the code changes we're making are hosted on OUR server so we're not technically distributing them to the public. So fuck you we have no obligation to make them open source".

Which is why AGPL exists so even server side code needs to be public. Since the application in question here is a backend service, it'll always be used server side and so any forks need to be open source.