What is the meaning of background app in Android?

Foreground refers to the active apps which consume data and are currently running on the mobile. Background refers to the data used when the app is doing some activity in the background, which is not active right now. 

This is due to the fact that whether they are active or not, apps consume data. They may be 

• checking for updates or refreshing the user content
• running ads in the background
• sending notifications

All these may be useful activities, but we do not want our data to deplete in the background without our permission and notice. We can restrict this background data depletion by using the settings tab in the mobile.