pstree
ProcessesPackage: psmisc
Processes as a tree, each child under its parent. It is how you see that an orphan node is the grandchild of a bash somebody closed, and not a service.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
pstree -p- Shows each process’s PID in the tree, without which you cannot act on any of them.