Android Developer Options background process settings

What exactly is a background process?

In Developer Options, there's are background-process settings such as...

Don't keep activities
 "Destroy every activity as soon as the user leaves it"
Background process limit
 "Standard limit, No background processes, 1, 2, 3, 4"
Background check
 (list of apps)
Show background ANRs
 "Display App Not Responding dialog for background apps"

What exactly is a "background process" anyway?
Is it as simple as just something you've run but you've not killed yet?

Or is it something more potentially nefarious, perhaps such as a Wi-Fi
scanning app that a Google API runs in secret without you knowing about it?