Spend Gold and enhance stones to increase the level of the rune, improving its stats and effects. As it increases in level, the amount of gold and enhance stones will gradually increase while the success rate decreases. The maximum level for each rune is +15. When an enhance attempt fails, your rune's stats won't be affected by you won't get back the enhance stones and gold used for the attempt.
You can enhance low-grade runes using the same method and even increase their grade (stars). However, consider doing this only for uncommon or rare runes that you want to use. Low-grade common runes should be salvaged for their enhance stones instead.