Marginalia are the notes you scribble along the sides of the text in a book. When you buy a novel at a used book store, you may discover the previous owner's marginalia among its pages.
The notes a student takes while reading often end up in the margins of the book. This marginalia might help the reader think critically and carefully about the text, or even act as a study aid later when there's a test on the material. Marginalia gets its name from the fact that it's written in the margin, which in turn comes from the Latin marginem, "edge or border."