As you may know, there are two optional fields in your profile:
My unicycles and Riding habits
From the user directory, which is found in the sidebar:
Searching with the “filter by username” input will also return results found in these two custom fields.
For example, if you search “Nightrider”, you’ll find users who filled in the My unicycles field with their Nightrider:
The interface and feature are a bit wonky, but Discourse knows about it; it will be improved over time