Here is the video version, if you prefer it:
Let’s introduce some concepts. A process is your operating systems container of a running program. Every application you run has its own process. Every process has its own memory and must not access other processes memory.
That being said, each process has its own unique process ID (also known as PID). Each process can be uniquely identified via its process ID.
Each process has an user associated with it – the user which instantiated the process. That user has an ID, and that users ID is remembered. Why is it remembered? So that only the user who initiated the process can affect its execution – start it, stop it etc.
There are in fact more than one user IDs, but in reality, nothing bad will happen if you simplify it and treat every process as having only one user ID. (Ward, 2014)
Hope you learned something new!
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 162-163