Groups are a really useful feature which let you follow discussions on particular topics. You follow a group's account to see discussions, and mention it to post to the group. More info:

➡️ https://fedi.tips/how-to-use-groups-on-the-fediverse/

If you're annoyed by high traffic groups, you can now combine them with the Lists feature to have groups in their own separate mini-timelines (and not your main timeline):

➡️ https://fedi.tips/how-to-use-the-lists-feature-on-mastodon/

Groups federate to all their followers, giving them a much wider reach than hashtags.