There were many different symbols used in the Norse world. Below are some of them and what they meant.
Yggdrasil [ig-dra-sil], The World Tree
Yggdrasil was what connected the nine worlds. Life flowed through it, in the roots that bore into Niflheim [niff-el-hame], to the utmost branches of Asgard [as-guard]. That was why it symbolised the interconnectedness of all life.
The Valknut was the symbol of Odin [oh-din]. It was the symbol of death, of souls of fallen warriors passing into the halls of Valhalla [val-hal-uh].
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The Horn Triskelion symbolised the three drinking vessels that contained the mead of poetry. According to the Poetic Edda, whoever drank the mead would become a poet. This was why the symbol of the three horns represented wisdom and inspiration (especially poetic inspiration).
Mjolnir [mi-ol-near], The Hammer of Thor
Mjolnir was a symbol of strength and protection. However, the hammer itself was more than just a weapon. Whilst it was responsible for crushing the skulls of many giants, it was also a symbol of purity, of leading lost spirits out of chaos and into the sacred realm.
Aegishjalmr [eye-jis-hal-mur], The Helm of Awe
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In battle, the Aegishjalmr would strike fear into the hearts of enemies. This is why Viking warriors used to paint this symbol on their foreheads.