Attacking the guard

This is universally true that attack the one first which guard the opponent. It is a very important chess logic which means if piece A guards piece B, then attack the piece A first and chances are that you win one or the other.
Generally there are some hostile pieces in the opponents closet, with the motive of of stopping your attack to the other crucial pieces.
for e.g. in figure, Black's Knight at King 5 is attacked by Queen and Bishop and defended by Queen and Bishop. So White removes the guarding Bishop in order to attack the Knight.