You can see the current Maintenance amount on the right side of the screen (1). You can also see Maintenance Production and Consumption over time in the Statistics screen (2).

Maintenance is a crucial global resource required by vehicles and buildings to stay operational. Without it, they will break down and stop working. The maintenance level of each entity is shown at the top of its window (1). You can hover over it for more details. In an emergency, broken entities can be instantly repaired using {0} (2).

In order to produce maintenance you need to build a {0} and supply it with the necessary materials.