https://developer.android.com/guide/components/activities/activity-lifecycle
As a user navigates through, out of, and back to your app, the Activity instances in your app transition through different states in their lifecycle. The Activity class provides a number of callbacks that let the activity know when a state changes or that the system is creating, stopping, or resuming an activity or destroying the process the activity resides in.
Within the lifecycle callback methods, you can declare how your activity behaves when the user leaves and re-enters the activity. For example, if you're building a streaming video player, you might pause the video and terminate the network connection when the user switches to another app. When the user returns, you can reconnect to the network and let the user resume the video from the same spot.
Each callback lets you perform specific work that's appropriate to a given change of state. Doing the right work at the right time and handling transitions properly make your app more robust and performant. For example, good implementation of the lifecycle callbacks can help your app avoid the following:
This document explains the activity lifecycle in detail. The document begins by describing the lifecycle paradigm. Next, it explains each of the callbacks: what happens internally while they execute and what you need to implement during them.
It then briefly introduces the relationship between activity state and a process’s vulnerability to being killed by the system. Finally, it discusses several topics related to transitions between activity states.
For information about handling lifecycles, including guidance about best practices, see Handling Lifecycles with Lifecycle-Aware Components and Save UI states. To learn how to architect a robust, production-quality app using activities in combination with architecture components, see Guide to app architecture.
To navigate transitions between stages of the activity lifecycle, the Activity class provides a core set of six callbacks: onCreate(), onStart(), onResume(), onPause(), onStop(), and onDestroy(). The system invokes each of these callbacks as the activity enters a new state.
Figure 1 presents a visual representation of this paradigm.
Figure 1. A simplified illustration of the activity lifecycle.
As the user begins to leave the activity, the system calls methods to dismantle the activity. In some cases, the activity is only partially dismantled and still resides in memory, such as when the user switches to another app. In these cases, the activity can still come back to the foreground.
If the user returns to the activity, it resumes from where the user left off. With a few exceptions, apps are restricted from starting activities when running in the background.
The system’s likelihood of killing a given process, along with the activities in it, depends on the state of the activity at the time. For more information on the relationship between state and vulnerability to ejection, see the section about activity state and ejection from memory.