The tabs at the top of your activity feed help to filter what activity from your groups are available.
Group Selector: The blue bar is a drop-down menu which lists each of the groups you are a member of, and allows you to switch between the activity feeds for each.
The Latest tab is the default and simply lists all response nuggets from most recently active to the least recently active. Active can mean new/updated responses, readers or stars/saves.
The Long tab filters for "long read" nuggets - documents that are longer than 1500 words and that readers have spent more than 5 minutes reading.
The Mine tab simply filters the feed to include only those responses that you have explicitly participated in by creating responses or replying to them.
The ★ (star) tab filters the feed for only those responses that you have ★'d. They act like a bookmark, making it easier to find them again later.
Once you have filtered the feed, you can then use search to filter further by text matches.