Marked as ticked (gnus-ticked-mark).

Ticked articles are articles that will remain visible always. If you see an article that you find interesting, or you want to put off reading it, or replying to it, until sometime later, you'd typically tick it. However, articles can be expired, so if you want to keep an article forever, you'll have to make it persistent (see Persistent Articles).

Marked as dormant (gnus-dormant-mark).

Dormant articles will only appear in the summary buffer if there are followups to it. If you want to see them even if they don't have followups, you can use the / D command (see Limiting).

Marked as unread (gnus-unread-mark).

Unread articles are articles that haven't been read at all yet.