http://jisho.org/words?jap=+%E7%B4%85%E8%91%89&eng=&dict=edict It describes the crimson-red color of leaves in autumn. Maple leaves also have this color. There's no exact English word for this, "red leaves", "autumn leaves", "crimson leaves", "maple" are all possible translations, depending on the situation. In your sentence, I'd translate it as "red leaves", because "In autumn there are autumn-leaves" uses the same word twice, and doesn't make it clear that it's referring to a color. I'd maybe even translate it as "because the leaves change their color to a beautiful red" or something like this.