Discourse is the platform the fiverr forum now runs on since the forum migration.
You don´t have to sign up there, unless you want to join the Discourse forum to talk about Discourse.
Just like if you read a Wordpress blog and comment there, you don´t have to join a Wordpress forum.
The links I think are just for information on things like badges or other Discourse options fiverr uses. If you want to read more about them or anything else about Discourse and about how it works, about the possibilities and functions etc. of Discourse, not of fiverr, you can read the Discourse forum, but you don´t have to sign up there, unless you want to take part in the Discourse forum discussions, which don´t have anything to do with fiverr.
edited for clarity:
‘don´t have anthing to do with fiverr’ apart of course from that the fiverr forum runs on that platform, so of course the functions of Discourse that fiverr uses, apply to the fiverr forum too, so in case you´d want to know specifics, like how certain things work (for example badges, or rights of moderators and so on), you can possibly find that information on the Discourse forum. Fiverr probably thinks it´s easier/makes more sense to link us to the Discourse site for information on e.g. badges than to write all that information up and put it on here.