The Kernel mode (Ring 0) is where critical system processes run, such as system call schedulers, heap memory management, and threading manegement etc. User-mode (Ring 1+) applications are not allowed to run in kernel-mode and vice versa
User-mode is where user applications are run. User-mode applications do not have access to the kernel mode and in order to communicate with the kernel mode, it has to raise exceptions/interrupts and wait until the kernel has been freed up and has validated that the user-mode function requesting access has passed all necessary checks