A system call in OS is a mechanism for an application to seek services from the kernel, the OS's fundamental component. It serves as an intermediary between user programs and privileged OS resources. Simply said, anytime a program wants to do something it can't do on its own, such as access hardware, manage memory, or create new processes, it makes a system call to the kernel, which then executes the necessary work and provides the result. This ensures secure and restricted access to sensitive resources.