A rune stone is a stone with runes inscriptions on it.
Typically, these come in a set of 24, called the Elder Futhark, and are used as a divination tool. However, there are stones in many places — mostly in Europe, but they’ve been found elsewhere — which have runes inscribed on them as in some kind of message.
It’s also possible to carve or paint bindrunes on stones, which in theory could also then be called rune stones.
The Elder Futhark runes are used for fortune-telling, devotion, and spiritual or magical practice.