Germany keeps a register of who lives where, and you are required to join it within two weeks of moving in. You book an appointment at a Bürgeramt, turn up with your passport, a completed form and a Wohnungsgeberbestätigung from whoever you live with, and walk out with a certificate called an Anmeldebestätigung.
Everything else in Berlin depends on that certificate. Your tax ID arrives by post a few weeks after it, and without a tax ID your employer withholds tax at the punitive emergency rate. Most banks want it. Every phone contract wants it. The residence-permit process wants it. People who arrive and cannot register can spend months working without being paid properly.
The problem for anyone subletting is that you cannot do it alone: the confirmation has to come from your Wohnungsgeber, the person providing the housing. In a sublet that is the main tenant, and they may be reluctant, either because their landlord does not know, or because they believe, wrongly, that registering you gives you a claim on the flat. It does not. Registration is an administrative record of where you sleep; it creates no tenancy rights whatsoever.
So the question “can I register at this address?” is worth asking before you view, not after you move in. A listing that says Anmeldung möglich is telling you the host already knows this and has said yes, and it is one of the most useful things a Berlin ad can say.