In Microsoft Azure Cloud, any user can own more than one subscription. It is the smallest payment unit, where you can configure payment method and configure resource ownership boundaries.
Every Subscription has own Billing Generation cycle. So you receive as much bills as you have subscriptions. There is exception for Enterprise Agreements, but this is out of scope of this text.
The important thing is you can receive for example three bills, each for two euros. It can be annoing, really.
So this is the reason for joining the subscription.
The only way for doing that is resouces migration to one choosen subscription from the others.
In the Internet, there is planty of questions about that, but responces are unclear or outdated.
Fortunatelly Microsoft has described it within documentation. It can be found here.
One thing is worth of notice. Every resource type has own migration policy, so possibility and limitations of given resource type depends on it.
Full list of resource types, what support migration you can find here.