Topics are a nice way of structuring newsgroups in a hierarchy. The nice thing is that you can use GroupParameters for Topics. This way, you can for example set the ScoreFile for an entire topic – for all the related newsgroups in the topic.
To toggle topic mode, use
‘t’. To switch it on permanently, add the following to your ~/.gnus file:
(add-hook 'gnus-group-mode-hook 'gnus-topic-mode)