In those days, busses in Montgomery had three sections- the first four rows were reserved for whites, blacks were in the back, and there was a middle section, where blacks could sit, until a white person wanted to sit in that section, when the blacks had to clear out.

(Apparently, it was not enough to have blacks in back, whites in front- the folks in charge back then didn't want the blacks anywhere near the whites- when the front rows were filled with whites and a black man wanted to ride the bus, he would have to pay his fare up front, then step back out of the bus and walk back around to the back door of the bus to get on, so that he wouldn't walk past the whites seated in front!)

Anyway, that day, the front section was full, and a white went to the middle section where four blacks were sitting. By the rules, they all had to leave and go to the back of the bus, even though only one seat was needed. Three of the four did so, but Rosa, the fourth, refused.

So it wasn't just that she refused to give her seat to a white man, she refused to empty the section so a white man could sit in a section without blacks.