Default [Q] why need to update with Karnel and what is OC ??

Right now I am android 4.1.1 and now I am going to update a ROM which is of 4.2.x but in every thread I have seen that first flash with kernel. Why we need to flash with karnel . In earlier many tym I have flashed many different ROMs but I never flash any kernel and without flashing kernel my ROMs are working fine in my device so why we need to flash kernel and what is OC mean over clock?? Thanks in advance..