An activity is a basic element of Focus. Activities can be created in the Focus main view
. To better understand activities, read the article What is an activity?
Each activity and indeed all content belongs to a cloud. The permission lists in a cloud define who can see and access the content.
Focus gets its name from its most prominent feature, focus. When focused, the app tracks your time and shares your focus with others in the cloud of the activity. Focusing also sets the context to which content is added from e.g. email or Dropbox. The FAQ
explains this in more detail.