CPU executes a program either in USER MODE or KERNEL MODE.
Most of the Kernel Mode processes will be, basic level core functionalities of system and such processes will have direct accesess to most of the available resources of the system.
Most of the User Mode processes will be of third party programs, which may be hazardes to a computet if we give direct access to resources.
So for a program running in user mode must have to uses Operating System APIs inorders to access resources with out any security breaches.
For such accesess the process in User Mode have to call OS functions with APIs provided, is known as system calls.