A rune stone with runes marked on it. Usually, 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 Europe, but they’ve been found elsewhere — which have runes inscribed on them as in some kind of a message.
It’s also possible to carve or paint bindrunes on stones, which in theory could also then be called rune stones. The most common use for the term though is for the 24 Elder Futhark runes which are used for fortune telling, devotion, and spiritual or magical practice.