|I don't know who made this|
In modern society, you can commit a criminal action if you break a law set out by the state forbidding certain actions. This would be like tax fraud, possessing drugs, or murder. In the iron/viking/medieval age things were different as criminal law wasn't as well developed.
So we have mainly two types of law in our society. Tort and Criminal. Tort refers to actions which harm someone or deprive them of something. Criminal refers to actions which harm the state or deprive them of something. Many crimes one can commit are both. For example if you murder someone you both deprive them of life and also break the law of the state.
In the iron/viking/medieval age instead of a state or society you instead had a king or ruler and there was much less overlap between the two. So if you killed a random peasant you wouldn't necessarily harm the king but you would greatly harm their family. So the family would likely take revenge but the king would likely not take an action. Instead of the king's men investigating the murder, the family would take actions against you either in the form of more murder or payment.
So for people during the iron/viking/medieval age it’s essentially just paying damages to someone else or the king or you specifically do something banned by the king. It’s up to you and your clan to atone for actions or damages made by you.
Here is a collection of the actions which would carry repercussions in the iron/viking/medieval age and how they would be solved:
Crimes vs ManMurder - Pay a weregild (man price) if their family doesn't try to kill you vengeance. Sometimes you'd also have to pay to the king as well.
Injury/Assault - So accidental injury wasn't a thing outside of Rome (I think), so you would have to pay a fine to the family or individual you harmed which would be proportional to the damage.
Larceny - You would have to either give the item back and may be assaulted from who you stole or pay a fine for what was stolen.
Crimes vs The KingMurder of a king's man - There is a much higher price here than a normal weregild. I'm sure he could also kill you for this but I don't know the specifics.
Crimes against the community - This is like arson, livestock mutilation, or disturbing the peace. Technically this is all the king's property and you're injuring him/stealing from him.
Forbidden action - If the king forbids something and you do it it won't end well.
On Crime PreventionSo a iron/viking/medieval age settlement wouldn't have police who would seek out crime and prevent it and solve it. You would have people who would patrol the town in order to keep order but it wouldn't be to prevent crime. If they came across a murdered body they wouldn't necessarily investigate but instead would maybe clean it out of the way. Further if someone had wronged you it was up to you to fix that wrong. You might petition your king for help but who knows what happen there? If someone stole from you you could possible get a thief-taker to track them down or you could possibly get someone to help your family gain vengeance. But most wealthy people would have guards walk the streets with them for protection instead of trusting the policing force to protect them.
Further if accused of a crime there wasn't much recourse for what you could do. In some soceities you simply needed witness to dispute it, in others you could try to have an ordeal to prove your innocence but the king would never have to prove your guilt. He's the king.
Other StuffSo you have to pay taxes and may have to become part of an irregular militia in times of war. Certain things may be taxed more as well, but generally you pay with items not with wealth, like a baker may pay tax with bread instead of coin. Foreigners had less legal safety, but their hosts could grant them protection or an extension of their family. Also this could go for orphans or such as well.
Some places also have "outlawing" which is when legal protections don't apply to you anymore, but that does come later historically. However, a similar concept that someone could be killed like a dog with no repercussions wasn't unheard of.