Other forms: apps
An app is a program that's designed to be used on your computer, tablet, or mobile phone. If you don't have a weather app, how will you know whether or not to bring your raincoat?
App is short for application. You can use it for any kind of computer application (like word processing or spreadsheets), but it usually refers to smartphones. If you have one, you've also got apps, for things like maps, banking, entertainment, to-do lists, and photo editing. Every so often, you'll also hear someone use app as shorthand for a restaurant appetizer: "I'd like to order some apps for the table, when you have a sec!"