Vættir is a word used in Icelandic mythology to describe different kinds of spirits or creatures that are believed to live in nature, such as forests, rivers, mountains or caves.
Imagine you are playing in the forest and you come across a little creature that seems to be made of leaves and moss. This could be a vættir of the forest. Or you could be swimming in a river and see a fish that looks much bigger and older than the others. This could be a vættir of the river.
Not all vættir are visible though. Some are invisible and you could only sense their presence, like a sudden gust of wind or a rustle in the bushes. They can be good or bad, helpful or mischievous, depending on their mood and how humans have treated them.
In Icelandic culture, people have traditionally respected and honored the vættir, leaving offerings or asking for permission before doing something that could harm their surroundings. Even today, some Icelanders believe in the existence and power of vættir and might still hold some of these traditions.