A constitution sets out the basic structure of government, defining what powers each branch holds, how they relate to one another, and often what rights citizens are guaranteed against the state itself. It functions as the foundational legal document that all other laws must remain consistent with at all times.
Because a constitution sits above ordinary legislation, changing it is usually deliberately harder than passing a normal law, often requiring a special majority, a referendum, or both of these together. That higher threshold is meant to protect fundamental arrangements from being altered by a passing political mood alone, however strong it feels at the time it arises.
Courts frequently play a central role in interpreting what a constitution actually requires in a given dispute, which is why judicial appointments and reasoning around constitutional matters tend to attract close and sustained public attention well beyond the specific case actually being decided at the time.