An app is a certain specific program which is downloaded, usually to a mobile phone or a tablet (but more and more commonly also on a laptop or computer), that sets out to do a certain job.
For example, Spotify is an immensely popular music streaming app, which can be used to access and play music from a device.
Since the boom of smartphones, the number of apps, or applications, has exponentially increased, and there is likely an app out there to help you out with anything.
There is a selection of free or paid-for apps available, from Google’s Play Store, Apple’s App Store, or from the Microsoft Store, which will be available on your devices (depending on which operating system your device uses).