An instrument cluster is the display behind the steering wheel that shows the car's speed, the fuel or battery charge and the warning lights. In older cars, it was a set of mechanical dials with needles. In most new cars, it is a screen. The dials on the screen are drawn by software, so the same screen can show a speedometer, a navigation arrow or a warning message.
The cluster does not measure anything itself. Other systems measure speed, fuel and engine faults. They send these numbers to the cluster over the car's internal network. In most cars this network is called a CAN bus. It is a shared pair of wires on which each computer sends short messages that all the other computers can read. The cluster only draws what the messages say.
Consider the moment you switch on the ignition. Every warning light comes on for about two seconds. Then they go off one by one, as each system reports that it is healthy. Say the airbag computer finds a fault. It sends a fault message, and the airbag light stays on. The cluster does not decide this. It lights whichever symbol the message names.
Because the cluster shows warnings, such as a brake fault or an airbag fault, it has a safety duty. If the screen shows the wrong speed or hides a warning, the driver can be misled. So cluster software has to meet strict rules, described under safety certification. A cluster is one of the electronic control units in the car.
The cluster is not the same as the central touchscreen. The infotainment system is for convenience, such as maps and music. The cluster is the screen the driver must be able to read in order to drive safely, so it is built more strictly.
The screen needs a display driver chip to light its pixels. Smooth dials and animations are drawn with the same kind of software as a video game, which is why some makers build clusters with a game engine. Electric scooter makers also write their own cluster software, to show battery charge and range. The cluster is one of the four parts of the digital cockpit.