You can follow a topic or forum. This will allow you to be notified of new posts and also gives you the ability to see a stream of new posts in your followed content activity feed.
To follow a topic or a forum:
Click Follow at the top of the topic or forum
Choose how you would like to be alerted:
One email per day with all new content from that day
One email per week with all new content from that week
Do not send me any notification