You’re wondering ‘what is a gauge cluster?’. Your dashboard, with the fuel gauge, the tachometer and the speedometer is a form of gauge cluster. A gauge cluster is any group of two or more gauges. So, a gauge cluster can contain two gauges, like an oil temperature and charge rate gauge, or you can add oil pressure to make a triple gauge set or cluster. Typically, the term gauge cluster only refers to aftermarket gauges in clusters of three that are mounted in an angled panel that mounts either on top of the dash or under the dash.
For more info, see Types of Car Gauges