System users are SSH/SFTP users that ServerPilot creates on your server. Every app belongs to and runs as one of these system users. You can log in to your server as a system user to deploy or update an app's code or view an app's log files.
By default, ServerPilot creates a separate system user for each app. This provides security isolation between your apps. Additionally, running each app under a separate system user makes debugging easier and provides a way for you to give your developers or clients access only to the apps they should have access to.