An Activity is a combination of multiple devices into a single mode. For example, when you are watching a show on your Set Top Box, you are also controlling the input of your television and the volume of your Sound Bar. You can combine this into an Activity called for example Watch TV.
In the Activity mode, you can assign groups of keys to different devices. The groups are the input key, the keys displayed on the screen, the keys except for the Volume controls, and the Volume Up, Down and Left. So when you select this Activity on the remote, you can control the relevant functions for the relevant device without having to swap devices.
