Kernel is an executable program that exists between SAP Applications and Operating System. It consists of executable program under the path — "/sapmnt//exe" for UNIX and \usr\sap\SID\SYS\exe\run Windows. It starts and stops application services such as message server, dispatcher, etc.
In Kernel upgrade, new .exec files replace old executable files in SAP system. To check the current version, use transaction code — SM51 > Go to Release Notes