Much like Mitteleuropa, it articulated Germany's foreign policy aim, prior to the First World War, of bringing the region under German domination.
The difference being that Mittelafrika would presumably be an agglomeration of German colonies in Africa, while Mitteleuropa was conceptualised as a geostrategic buffer zone between Germany and Imperial Russia to be filled with puppet states.
These plans were arguably made to be used only as a last resort to appease Germany in case she threatened to disrupt the balance of power in Europe.
However, since German foreign policy interests were in subsequent years mainly directed at gaining mastery in Europe itself, and not in Africa, they were eventually shelved.
The full realisation of Mittelafrika depended on a German victory in the European theatre of the First World War, where Britain would be forced to negotiate and cede its colony of Rhodesia to Germany when faced with a German-dominated continent across the English Channel.