Communities are large public channels where Session users can congregate and discuss anything they want. Communities, unlike other features in Session, are self-hosted and thus not fully decentralised. Someone has to run a server that stores the communities message history.
Additionally, because community servers can serve thousands of users, messages are only encrypted in transit to the server rather than being fully end-to-end encrypted.
Users seeking smaller group chats with greater privacy are encouraged to use Session's groups feature.
You can learn more about communities and how to set one up here.