Harry recognizes some of the carvings as runes. So what are runes, and why would they belong on a pensieve?

Runes were the first alphabet of the tribes of northern Europe, used in Britain, Scandinavia, and Iceland. They appeared about the third century AD and endured more than a millennium. The first six letters were f, u, th, a, r, and k, so the alphabet is sometimes called “futhark.”

Rûn is Gaelic for “secret,” and helrûn means “divination,” suggesting that the alphabet was used in magic rituals. They are said to come from the gods themselves. Odin hanged himself for nine days from Yggdrasil, the great ash tree that holds earth, heaven, and hell, to earn knowledge of the runes. Odin’s sacrifice to gain knowledge for humankind symbolizes how greatly learning was valued. In fact, when runes were first used there was little distinction between scholars and wizards. Runes were sometimes used for divination, and in recent years this practice has become popular again. Their symbols are read the way some fortune-tellers read tarot cards.

In Phoenix, Hermione takes Ancient Runes and worries that she confused two of them, ehwaz and eihwaz, on her exam. She did. The definitions Rowling mentions are actually used by people who cast runes to tell fortunes: ehwaz signifies partnership and eihwaz signifies defense.