Isa is the Rune of stillness and consolidation.
Anglo-Saxon Rune Poem
Is byþ oferceald, ungemetum slidor,
glisnaþ glæshluttur gimmum gelicust,
flor forste geworuht, fæger ansyne.
Ice is very cold and exceedingly slippery;
it glistens, clear as glass, very much like gems,
a floor made of frost is fair to see.
Norwegian Rune Poem
Ís köllum brú bræiða;
blindan þarf at læiða.
Ice we call the broad bridge;
The blind man must be led.
Icelandic Rune Poem
Íss er árbörkr
ok unnar þak
ok feigra manna fár.
Ice is bark of rivers
and roof of the wave
and destruction for doomed men.
Ice was something that Northern Europeans were very well acquainted with. They were very aware of its dangers but also of its usefulness and beauty. A frozen lake could give you access to the people living on the other side without the need to use a boat or to walk around. There are quite a lot of finds of Viking bone ice skates that were used to quickly cross frozen bodies of water. In that regard ice could serve as a broad bridge in a very practical sense. Especially in early spring, these bridges could become rather dangerous and took many doomed men into their watery depths beneath the thinning ice.
In a magical sense, Isa can be used to slow things down. It is a useful tool when trying to quiet our mind, to freeze our hectic thoughts and to find that inner stead of stillness. It can also serve as a bridge to reach foreign shores that we usually cannot reach and to consolidate knowledge or energy, as ice is the densest form of water.