The mobile/tablet app includes video and audio lessons which will be automatically downloaded to the device after installation (only when connected to WiFi and only if there is enough space available). The total size of the media files is about 1GB.
When the device does not have enough space available or there is no WiFi connection, the lessons will be streamed over any available internet connection.
To free up or save space on your device, please navigate to the Account tab -> My Settings (select it from the orange menu on the top) -> Downloads page, and turn off the Offline Media Playback feature. This way, the offline content will be removed from your device, and for further media playback, you will need an active internet connection.