The group feed is the main page of a group. It shows all the activity happening.
While the main way of group interaction is via Discussions, you can use the activity feed to share links or videos.
If you are a member of the group, you can share updates with the group on the Feed tab. Click in the box Share a link, video, or picture… and enter what you want to say. You can change text formatting (bold, italic, etc), add images, documents, GIFs, or emoticons.
When you are done, click Post Update and your update will then appear at the top of the feed. Other members of the group will be notified and can interact with your post by reacting to it or commenting on it.
For quickly checking for updates posted in the past, there is a simple search feature available near the top of the feed.