One of the most important tasks in Frostpunk is keeping the population happy. The morale of your people is represented by two indicators: the Discontent and Hope - both located on the bottom of the screen. Each new law and new building influences at least one of them.
If the Discontent reaches 100% or near, or when the Hope reaches 0%, you will be informed that things in the city are dire and desperate. You will also get a list of proposed solutions. The first two require the right buildings unlocked in the Book of Laws of Order or Faith. The third one gives you an ultimatum: modify one of the indicators by 15%. You have two days to do that - but it's definitely more challenging than the remaining two solutions.
If you don't manage to fulfill your promise, people will mutiny. You will be banished, and the game will end.