explain this para ( for a capacitor to store latrge amount of charge without leaking, its capacitance should be high enough so that the potential difference and hence the electric field do not exceed the break-down limits.

It means that the capacity of the capacitor must high  to store more energy (the capacitor  should have the capacity to store large amount of energy) . Due to which the potential difference between two electrodes and electric field wire does not break down due to high energy ,as the capacitor will absorb all extra energy .