Federalism is the art of being one country and many at once. It splits a nation's sovereignty between a central government and its regions, each holding constitutionally protected authority of its own, some powers exclusive and many shared, so that a large and varied people can share a single state without dissolving into one ruler's grip or a quarrelling heap of statelets.
In the United States, cannabis can be lawful under Colorado's law and a crime under Texas's at the same time, even while it stays illegal under federal law. Schooling, policing, taxes, and the death penalty all change the moment you cross a state line. This is not a flaw in the system. It is the system. Federalism is the deliberate choice to divide a country's authority between a national government and its regions, so that fifty different sets of laws can live under one flag. It lets a huge and clashing people stay one nation, and it is also why the same act can be a right in one place and a punishment in the next.