The Android documentation defines an activity as “an application component that provides a screen with which users can interact in order to do something, such as dial the phone, take a photo, send an e-mail, or view a map. Each activity is given a window in which to draw its user interface. The window typically fills the screen but may be smaller than the screen and float on top of other windows.”
Android applications consist of one or more activities loosely coupled together. Each application will typically have a “main” activity that can, in turn, launch other activities to accomplish different functions.