Community members have the option to subscribe to any board, topic or message in the community which will then trigger a notification email to them for any new responses. There are various preferences around notifications that users can set for themselves or be set as default by the community admin, such as automatically subscribing to forums in which they post or the frequency at which they receive notifications.
Here is an article which you may find useful to gather more information:
On a slightly different note, privileged members also have the option to send bulk emails, if for example there is a new feature they wish to communicate to members.
I hope this answer and the articles I've linked you to are useful. Please let me know if there's anything else I can help with.
If you are reerring to a more general meaning of "topic" (not a discussion thread) then Lithium offers a method via search as well. E.g. if you are interested in new discussion around the topic "Skype" you can find a link to subscribe to search results via an RSS feed: