A Low bounce rate means that users are finding your Discourse site useful and they are spending a lot of time in it exploring the great content in it!
So it is a good news that you have such a low bounce rate which is recommended for sites to rank higher on SERPs. We have a bounce rate of 35% for this forum because users are spending a lot of time here and thus giving us more pageviews.
Bounce rate of a website = Total number of bounces across all the pages on the website / total number of entrances across all the pages on the website
A high bounce rate is what you must not have. A high BR means that users are finding your site less useful and therefore closing it after a single visit.
For Bounce rates a simple rule of thumb is:
Anything bounce rate below 50% means well done and anything above it means get serious!