basically... you (as a cop) are driving doewn the road, see a black male in early 20's driving a lexus and decide "hey, he's a drug dealer" and pull him over. turns out the guy is out taking his Dad's car to get washed. He did nothing illegal, he committed no crime, and was stopped, detained, (potentially harassed)and interferred with for the fact that he "looked" like a drug dealer. How did he look like one, he was young, black, and in a car "outside his range". Don't like the black analogy, okay...now you are white guy, mid thirties, driving a rusty pickup truck in neighborhood of $300,000 houses, police stop you because "you look like you are casing the area"...now you did nothing illegal, committed no crime, and were stopped, detained, (potentially harassed)and interferred with for the fact that you "looked" like a criminal. and actually you were on the way to do yard work.
Once you are stopped for "no apparant reason", and hassled when you did nothing wrong, just look out of place...you'll understand why it's wrong.