Consider this scenario: A process forks, the parent and child are alive and running their individual code paths in parallel. Let's say the parent's PID is 100 and the child's is 102, implying the child's PPID is 100 of course.

The parent process, for whatever reason, dies. The child continues on without any trouble, except for a side effect: The moment the parent (PID 100) dies, the child's PPID (100) is now invalid! Thus, the kernel intervenes, setting the child's PPID to the overall mothership—the ancestor of all user space tasks, the root of the process tree—the init, or on recent Linux, the systemd, process! It's PID is, by venerable Unix convention, always the number 1.

Terminology: the child that lost its immediate parent is now said to be re-parented by systemd (or init), and its PPID will thus be 1; this child is now an orphan. 

We notice that the child's PPID value abruptly changed; thus, the systems programmer must ensure that they do not depend on the PPID value being the same (which can always be queried via the getppid(2) system call) for any reason!