I wasn't sure about this term the first time I read To Kill a Mockingbird, so I had to research the term myself. "Snow-on-the-Mountain" is referred to at the end of Chapter 11 when Jem receives a posthumous gift from Mrs. Dubose shortly after her death,
... a white, waxy, perfect camellia. It was a Snow-on-the-Mountain."
A Snow-on-the-Mountain is a rare type of camellia, also known as the Christmas Camellia (or Camellia sasanqua). Native to Japan, it is Mrs. Dubose's prize variety and usually found only in hilly, coastal evergreen forests in Asia. The camellia serves not only as a final remembrance from Mrs. Dubose, but also to remind Jem that his attempt to destroy the plant was unsuccessful: that unless you kill it at the roots, it will still survive. The camellia also represents racism, the white flower symbolizing Mrs. Dubose's white supremacist attitude toward Negroes; and how racism also must be destroyed at the roots or it, too, will continue to grow.