It's basically just like a Facebook group or a sub-reddit. A collection of posts of a certain type of content to help with discovery and finding the stuff that interests you.

So now along with following people, you can follow Communities and you have two feeds, one for the people you follow, and one for the Communities you follow. It's really useful, because you can get exposed to people you don't follow, that are posting in a community you follow, as well as people who don't follow you getting exposed to your work because they follow the community.

On PeakD, if you go to OnChainArt's page, there is a blue button at the top that says "Write A Post", just click on that, and that's it :) Similar process on hive.blog, but the interface is pretty bad, I'd suggest using PeakD.

You can also post into the community using any frontend by using the community tag as the first tag. For OCA it's "hive-156509" That part can be a bit confusing for people but if you use the web interfaces, that's not necessary.