Mode setting is an operation that sets the display mode including video resolution and refresh rate. It was traditionally done in user-space by the X-server which caused a number of issues due to accessing low-level hardware from user-space which, if done wrong, can lead to system instabilities. The mode setting API was added to the kernel DRM framework, hence the name kernel mode setting.
The KMS API is responsible for handling the frame buffer and planes, setting the mode, and performing page-flips (switching between buffers). The KMS device is modeled as a set of planes, CRTCs, encoders, and connectors as shown in the Display Software Stack figure in the Display section. The figure also shows how the driver model maps to the physical hardware components inside the HDMI Tx display pipeline