The activity feed of a spot is the common feed. Everyone in the spot can view posts that are published in the activity feed. As a user, you can also post in the activity feed by following the below steps:
1. Go to the Activity tab and click on the + icon (bottom right) 
2. Type in your post and click on the post icon to publish your post
-You can tag user(s) by using @ followed by the name (@Tina Lewis)
-You can use hashtags for better search and visibility of your post by using # followed by the tag (#GoodMorning)
-You can include various media files like documents, excel sheets, images, video files, gifs, etc.
-You can use the rich text editor to enhance your text (bold, italics, etc)
Check out the article - Post in feed group to know how to publish a post in a group
You can react and comment on any post on the feed. You can include a media file, hashtag and can also tag another user in your comment.