User navigates between different screen or app, it goes through different states in their lifecycle. So an activity lifecycle consists of 7 different methods of android.app.Activity class i.e :
onCreate() : In this state, the activity is created.
onStart(): This callback method is called when the activity becomes visible to the user.
onResume(): The activity is in the foreground and the user can interact with it.
onPause(): Activity is partially obscured by another activity. Other activity that’s in the foreground is semi-transparent.
onStop(): The activity is completely hidden and not visible to the user.
onDestroy(): Activity is destroyed and removed from the memory.