Any application running on Linux may request services like :
Inter process communication I/O services Creating a new process Accessing system hardware etc
Note: The above list is not exhaustive.
The services listed above are all handled by Linux kernel. So, any application in Linux that requires any of these services has to request the Linux kernel to handle the request on its behalf. The request is handed over to kernel by calling corresponding system calls. System calls act as an interface between application and kernel.