Every running program on a Linux system is a process — an instance of an executable loaded into memory with its own PID (process ID), memory space, open files, and environment. The kernel's scheduler decides which processes get CPU time. Processes form a parent-child tree: PID 1 (systemd) is the root, and every other process was spawned by a parent via fork()+exec(). When you type a command in bash, bash forks a child, the child execs the command, and bash waits. Understanding this tree — who spawned whom, what state each process is in, how to signal them — is the foundation of every troubleshooting and performance question on the RHCSA.