The document discusses system calls in Linux. It defines system calls as service points for user programs to request services from the kernel. It describes how system calls provide an abstracted hardware interface, ensure security and stability, and make process management easier. The document contrasts system calls with library functions, noting that system calls execute in kernel space with special privileges while library functions are ordinary user space functions. It also covers how system calls are implemented and executed via wrappers and describes tracing system calls using strace.