Storage (per app)
Storage in the App info shows the amount of storage on your device used by this app. Storage used by an app can be broken down into storage used by the app itself, its data, and its cache. If only a total is shown, tapping it should bring up a screen showing the detailed breakdown, along with the Manage space and Clear cache options.
If the app is shown as occupying 0 bytes, this may indicate the app code only exists in the system partition pre-loaded on your device.
Some apps will have a Manage space button, taking you to app-specific options for managing or freeing the data stored by the app.
Any storage used by an app that is classified as Cache may be safely wiped without affecting the operation of the app, and is used only for speeding up app operation or saving the need to download data that has already been downloaded.
You can often free quite a lot of storage space on your device by clearing app caches, but apps will start to accumulate more cache again with further use.