In Clear it means that if you set:

MCLR=ON > you must have an external resistor on this pin and you cannot use this pin as an i/o

MCLR=Internal >you don't need any external resistor on this pin an you can use this pin as an i/o... usually Input.

for the blabla in the datasheet... it means to me that when MCLR=ON, if you pull down ths pin to ground, the internal or external oscillator will be disable. This is why your program will stop or not boot.