A system call is a way for programs to request services from the operating system's kernel. This allows applications to perform functions like reading files, creating processes, or communicating over a network. System calls act as an interface between user applications and the underlying hardware, ensuring that programs can safely and efficiently access system resources. When a program makes a system call, it typically switches from user mode to kernel mode, where it has higher privileges. This transition allows the operating system to manage resources and maintain security, preventing user applications from directly interacting with hardware or other critical system components.