the process control block
Think of a hospital keeping one chart per patient: name and ID, current condition, medications, who the doctor is, what tests are pending. The staff can put a patient aside and pick them up later because everything needed to resume care is on the chart. The process control block, or PCB, is the operating system's chart for one process: a record holding everything the OS needs to know about that process so it can pause it and resume it correctly.
A PCB typically stores the process identifier (PID); the process state (running, ready, waiting, and so on); the saved CPU registers, including the program counter, so the process can be resumed exactly where it left off; memory-management information such as where its address space lives (base/limit or page tables); a list of open files and other I/O resources; scheduling information like priority; and accounting data such as CPU time used. When the OS does a context switch, it saves the running process's registers into that process's PCB, then loads the next process's registers from its PCB; the PCB is precisely what makes pausing and resuming possible.
The PCB is the concrete embodiment of a process inside the kernel. The OS keeps these structures in a process table and uses them to track and manage every process in the system. A useful mental check: anything the OS must remember about a process across the times it is not running has to live in the PCB (or structures it points to), because everything in the CPU's registers is overwritten the moment another process runs.
Just before switching away from process A, the OS copies A's program counter and registers into A's PCB. Hours later, to resume A, it copies them back from the PCB into the CPU, and A continues as if it never stopped.
The PCB is the saved snapshot that lets a process be paused and resumed.
The PCB lives in kernel memory, not in the process's own address space, so a process cannot read or corrupt its own PCB; that separation is part of protection.