For example, if I call a white guy a "cracker". Other than maybe getting his feelings hurt, there's nothing else that harms him. I called him a name but what other effects does it have? None.
But, if someone calls an undocumented immigrant an "illegal", there is a lot of power behind that word. It upholds a system where this person is a criminal, doesn't belong and is all around a bad person. And the use of this word can and has influenced people to hate them, discriminating against undocumented immigrants and creating laws and legislation to get rid of them and doing anything they can to prevent these people from succeeding.
The word holds so much power and influence, there is no way one can compare it to cracker. I have no power to take away from this white person, but this white person has all the power in the world to make my life as difficult as it can be. This is what racism is. It's power combined with prejudice.
This is why reverse racism is a myth. My words and actions cannot touch the power system that you have in place. This is why affirmative action is so frowned upon. It disrupts the system that allows whites to succeed over groups of color, threatening the power structure that they hold.