You know how you and your friends with different email services can still email each other?
Now imagine all the messages you "email" back & forth are instead messages you've posted on a message board, and anybody can read & reply
To address another complex matter if they ever ask:
You know how your email service can help sort or block spam in your incoming email? Hosts of one message board service can filter/block other particular people's/services' messages from showing up to your feed, too