From the Android Developer documentation:
"When the activity enters the Resumed state, it comes to the foreground, and then the system invokes the onResume() callback."
What means "comes to the foreground"? Respectively: What's the opposite of running in the foreground?
Running in the Foreground means your app is currently Fully Visible on your device, you can see it and interact with it and it will respond to you right away.