What is the idrottir and why is it so important?

Idrottir is a list of skills one possesses, used to contribute to one's blood-price in the Viking Age and thereabouts. They could be physical skills, or academic, such as reading and writing, use of a sWord, riding of horses, herding of sheep, player of Chess, etc...

The more skills acquired, the higher the blood-price, and the higher one could (ideally) rise in society.