The neighbors hear a rumor that Mr. Radley has shot at "a Negro" in his collard patch. This conclusion has most likely been made because their thinking is that only a black person would be so poor that he would consider stealing collards. For, if white people want them, they can simply grow them.
Mr. Radley himself did not say that a black person stole into his collard patch. He said, according to Miss Stephanie, "if anybody sees a white n----r around, that's the one. Says he's got the other barrel waitin' for the next sound he hears....be it dog, n----r, or--Jem Finch." So, clearly, he saw Jem as he peered into the window. Nevertheless, the gossips take the story and shade it to their whims.
This re-interpretation of what has occurred in the night is significant because it foreshadows the propensity of people to quickly jump to conclusions, as they are so ready to do at the Tom Robinson trial further on in the narrative.