Displays are geared for a specific resolution. what is this resolution called?

Its Native resolution. The maximum resolution of a monitor or printer. In a flat panel display, the native resolution is significant because, unlike older CRTs, which could display several resolutions (800x600, 1024x768, etc.) at the same quality level, the highest resolution (native resolution) of a flat panel generally provides the best quality, especially in earlier monitors.

However, as screen pixels (ppi) increasingly become denser, this is no longer a hard rule. In order to display everything larger on a big screen (27" and higher), a 4K or 5K monitor can be used. The screen resolution can be set lower than the maximum native resolution, and the quality is excellent