Allemannsretten - literally everyone's right - is the Norwegian legal principle that gives any person, regardless of land ownership or nationality, the right to walk, ski, cycle, ride, swim, paddle, camp and forage on uncultivated land in Norway. The principle has roots in medieval Norse common law but was codified in modern statute in the Outdoor Recreation Act (Friluftsloven) of 1957. It is among the most permissive outdoor-access regimes anywhere in Western Europe and has no direct equivalent outside the Nordic countries.
What it means in practice: you can walk freely across uncultivated land, including private forest and private mountain, provided you do no damage and stay 150 meters from inhabited buildings. You can pitch a tent for one or two nights anywhere in the open countryside, again at 150 meters from a building, without seeking permission. You can pick wild berries, mushrooms, flowers and herbs for personal use. You can paddle, swim, ski and cycle with the same freedoms.
The corresponding responsibility - the part of the law that international visitors sometimes underestimate - is real. You are required to leave no trace, to respect grazing animals, and to follow the Norwegian Mountain Code (fjellvettreglene) on serious mountain trips. Between 15 April and 15 September there is a national prohibition on lighting open fires in or near forests except in designated fire pits, and that rule is taken seriously.
For the visitor, allemannsretten is one of the things that fundamentally shapes how Norwegian outdoor travel feels. Trails that elsewhere in Europe would be private or paywalled are simply open. The DNT hut network sits on top of this access right. The whole concept of a long fjell-to-fjord friluftsliv tradition depends on it. It is also why Norwegian hikers tend to be unusually disciplined about leave-no-trace ethics - the freedom is wide, and the cultural expectation is that you do not abuse it.