What is chapter 13 about in Harry Potter and the Philosopher's Stone?
Harry, Ron and Hermione find out who Nicolas Flamel is and that he is the only known maker of the philosopher's stone. They correctly guess that the stone is what Fluffy is guarding.
Harry plays his second Quidditch match, Gryffindor v Hufflepuff with Professor Snape as the referee. Gryffindor win the match in under five minutes. After the match, Harry follows Snape into the forest and overhears him speaking to Quirrell. Harry thinks that Snape is forcing Quirrell to help him steal the stone.