For nine days and nine nights, Odin hung from the world tree pierced by his own spear. He sacrificed himself onto himself – the highest of the Gods. He did this to learn the wisdom that would give him power in the nine worlds. Nine is a magical number in Norse Mythology, after all there are nine realms of existence in the Norse Cosmos. Odin learned 18 magical runes. (Source: Rúnatal, a section of the Hávamál)
So what are the Runes? They are a gift to mankind. These sacred letters originally were carved into stone. Each letter has a metaphorical meaning as well as divination abilities. I suggest doing much research to completely know the runes. Reading this page alone won’t help you learn everything. After all, Odin had to hang from a tree for nine days pierced by a spear. He never said learning everything there is to know about the runes would be easy.
The graphic below may be helpful: