App is short for application, and, in the context of phones, refers to a software program.
An app is a distinct unit of software that provides specific functionality. Phones may come with some apps built-in, and may allow others to be downloaded and installed by the user. Some apps are free, while others may cost money. Modern smartphones have an app store to help find new apps to install.
Examples of apps include: to-do list, game, drawing tool, calculator, email, web browser, game, photo editor, and a weather forecast tool.
Smartphones can run much more powerful apps than non-smartphones (feature phones.)