The article does a good job breaking it down, but a short example is if you want to post about technology, you have to choose one of those communities to post to.

You can cross post or repost in the other communities but then any discussion gets fragmented, people see the same thing multiple times in their feed, only engage with one, and likely not the same one as somebody else.

On the other hand only posting in one community could significantly reduce engagement if you choose poorly.