Mode setting is a software operation that activates a display mode (screen resolution, color depth, and refresh rate) for a computer's display controller by using VESA BIOS Extensions or UEFI Graphics extensions (on more modern computers).
The display mode is set by the kernel. In user-space mode-setting (UMS), the display mode is set by a user-space process.
Kernel mode-setting is more flexible and allows displaying of an error in the case of a fatal system error in the kernel, even when using a user-space display server.
User-space mode setting would require superuser privileges for direct hardware access, so kernel-based mode setting shuns such requirement for the user-space graphics server.