A system call is a way for a program to request a service from the operating system's kernel. This allows applications to perform functions like reading files, creating processes, or communicating over a network. By using system calls, programs can interact with hardware and manage system resources safely and efficiently. When a program makes a system call, it switches from user mode to kernel mode, where it has higher privileges. This transition ensures that the operating system can control access to hardware and protect system stability. Common examples of system calls include open, read, and write.