In a "Wohngemeinschaft" or WG, several persons share a flat. In most WGs everyone has their own room, but the bathroom and kitchen are shared. Finding a room in a shared flat usually is somewhat easier than finding your own flat, because you do not have to convince a landlord, but only the member(s) of the flatshare. Furthermore, a room is cheaper than a whole apartment.
If you receive financial support from the Job Centre or Social Welfare Office, you will need to submit to the Office both your sublease agreement and the primary tenant's rental contract. Many administrative assistants also want to see a confirmation from the landlord which proves the primary tenant is allowed to sublet the room.