Hey yeah I understand how it can be confusing! I started as a content creator too and have had to learn a lot of the tech stuff along the way
Discourse is free software, but it has to be hosted somewhere. Similar to Wordpress, if you’re familiar with that at all — it’s open source, so anyone can use it for free, but it has to be installed on a server somewhere, and that’s the part that costs $ and requires setup.
Discourse (the company who creates the software) will host it for you and manage all the tech stuff but it is a bit pricey: https://payments.discourse.org/pricing
However if you read the FAQ further down that page you’ll find other options — you can either install yourself for free (fairly straightforward but does require a bit of technical proficiency) or you can buy a $99 install, and after that pay just the hosting fee, which would start at $5 a month on Digital Ocean for example.
Guessing you’re seeing the $20/month option at Plans and Pricing - DiscourseHosting which is yet another option! That’s hosting with a third-party company, and seems to fall somewhere in between the top of the line official business hosting level and the DIY method. So that might be a good way to go if you can swing $20 / month.