It is the first program that runs after the bootloader.
It also acts as a bridge between hardware and application.
CPU can run some instruction only on kernel-mode also called Privilege instruction, thus it changes to kernel mode from user mode.
What is Microkernel?
A microkernel is the classification of the kernel. User services and kernel services are kept in different addresses making easy access and providing isolation to both the services, this also reduces the size of the operating system.Also if a new service is added there will be no change in the kernel even if there is a failure.The Microkernel is responsible only for important services like: