In Android, an activity is referred to as one screen in an application. It is very similar to a single window of any desktop application. An Android app consists of one or more screens or activities. 

Each activity goes through various stages or a lifecycle and is managed by activity stacks. So when a new activity starts, the previous one always remains below it. There are four stages of an activity.

The Activity Lifecycle  : https://www.geeksforgeeks.org/activity-lifecycle-in-android-with-demo-app/