why does a crickter lower his hands while catching a ball?

The cricketer lower his hand thus to reduce the speed of the ball slowly to zero instead of immediately reducing it to zero.The force that the hand feels can be reduced by moving in direction of the ball. this is because time will increase,while the impulse remanis constant.

dear alish 
its the simple concept of momentum and the newtons third laws of motion . 
if the cricketer will catch the ball immidiately then its speed reduced to 0 at a sudden point of time , so, that might hurt the cricketer’s hand . 
so, he lowers his hand to reduce it slowly . 
this decrease the rate of change of momentum , and hence the force exerted on his hand is also decrease , thus it saves the hand from getting hurt . 

HOPE IT CLEARS 
REGARDS