System call
How a program asks the kernel to do something.
User programs cannot touch hardware directly. When a program needs to open a file, allocate memory or send a network packet, it makes a system call: a controlled entry into the kernel, which validates permissions and performs the operation.
Most commands are thin wrappers over a few: cd is chdir(), rm is unlink(), mv is rename(), mkdir is mkdir(). Knowing the underlying call often explains a command's exact behavior better than its manual page.
See it in practice
$ strace -c ls 2>&1 | tail -12Counts the system calls a simple ls makes. Every file it lists went through openat, getdents and friends.
Where you'll meet it
The core of the operating system, managing hardware and processes.
rmDelete files and directories, and understand -r, -f and why there is no trash bin.
mvMove files between directories and rename them: the same operation in Linux.