When something patters, it makes a light, rhythmic, tapping sound. On a rainy night, you might love lying in bed listening to the rain patter on the roof.
You can describe the way rain patters, or the way children's feet patter down the hallway on Christmas morning. The light, quick sound itself is also a patter: "Listen to the patter of sleet on the window!" Informally, a salesman's pitch can be called patter as well, from an older meaning, "talk rapidly," from pater, "mumble prayers rapidly." The tapping kind of patter has a different root, which it shares with pat.