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.