Hi,
if you see a cloud icon with an arrow (similar to this one:
) it means that the file or activity can be downloaded for offline usage. If you click that icon, the file or activity will be downloaded. Once the file or activity is downloaded, it should display this icon instead: 
if you see a cloud icon with an arrow (similar to this one:
) it means that the file or activity can be downloaded for offline usage. If you click that icon, the file or activity will be downloaded. Once the file or activity is downloaded, it should display this icon instead: Please notice that those icons are hidden by default in courses (to save network usage and speed the app, calculating the status of each activity isn't simple). They can be enabled by opening the top-right menu in the course page (the 3-dots button) and enable "Show download options".
Users will be able to play downloaded activities in offline (e.g. a quiz or a SCORM). When the user is connected again, all the data will be sent to Moodle. Please notice that the app must be running (or in background in Android) in order for this to happen, if the app is "dead" the data won't be synchronized until the user opens the app again.
The data that the user generates while offline is sent to Moodle, but please notice that the dates stored in Moodle logs will be the ones of the synchronization, not when the user did the activity. E.g. if as user answers the quiz offline on July 28 at 16:00 but synchronizes it on July 29 at 10:00, the logs will say that the user answered the quiz on July 29 at 10:00.
Cheers,
Dani