Groups are a really useful feature which let you follow discussions on particular topics. You follow a group to see discussions, and mention the group to post to it. 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 Mastodon's Lists feature to keep them in their own separate mini-timelines (and not in 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.