Word
Process
A process is one running copy of a program, which your computer numbers so it can be told apart from the others.
Also called: process id, PID, a running program
A program sitting on your disk is a file. The moment it starts, your computer makes a running copy of it and gives that copy a number, the process id. That running copy is the process.
You'll meet the number in one place: an error saying something is already running, with a number in brackets. That's the app you started earlier, still going in a window you've lost. Nothing is broken. Find the window and close it, rather than starting a third copy on top of the second.