A process is a program that is currently running on a computer. When you open an app, the computer starts a process to do its work and keep track of the resources it needs.
Think of a process like one worker at a desk, with its own workspace and assigned tasks. Several processes can run at the same time and take turns using the computer's processor.
If a process stops or gets stuck, the app may stop responding. Closing a process ends that running instance, but it does not always delete the app or its saved files.