Automatic control hookup means that when the wing or tail are fully inserted into the aircraft fuselage, that the control rods for the airlerons, flaps, speedbrakes, elevator, etc., are all guaranteed to be fully and properly connected to the control handles inside the cockpit.
Without automatic control hookups, the control rods coming from the wing or tail control surfaces had to be manually connected, one by one, to the control rods coming from the cockpit. This system led to many accidents, when pilots forgot to connect a control rod during assembly, and they didn't find out until the glider was launched:
Another type of manual control rode failure is associated with the L'Hotellier connector, which would seem to be fully connected, but would fall apart, even after a positive control check.
Automatic control hookups are designed to reduce, if not eliminate, these kinds of problems. Needless to say, even with automatic control hookups, a positive control check must still be performed before every flight.