Complex sentence: If all the input voltages are low (input logical "0s"), the voltage drops across the diodes are zero; these diode switches are open and the input sources (grounds) are disconnected from the output.
Split into simpler sentences: If all the input voltages are low (input logical "0"), the voltage drops across the diodes are zero. These diode switches are open and the input sources (grounds) are disconnected from the output.