A postscript is text added at the end of a book or other document. Your textbook is so long that by the time you finish it, you're too exhausted to read the postscript.
Postscript comes from the Latin word postscribere, with post meaning after and scribere meaning to write. A postscript especially refers to a note added after the signature of a letter. In this use, it's often abbreviated as P.S. A mother's favorite thing might be getting a letter from her kid at camp that says, "I love you, Mom," especially if there is a postscript adding hugs and kisses, written this way: "P.S. XOXOXOXOX."