Umm....wow
I'll give you the real short version. Maybe someone else would like to give a more detailed answer.
When you bet you have two ways to win.
1- By showing down the best hand
2- By making your opponent fold
When you check and call (i.e passive play) you have only one way of winning: By showing down the best hand.
When you let others bully you and fold to their aggression, you are giving them pots
When you only call rather than raising , you are winning less money in the pots that you win, and you aren't finding out where you are in the hand, so you are losing more money on the pots you lose.