I have explained how cross posting means in one of my posts. You can find the link to that at the bottom of this post. But I'm going to explain what you don't understand. When you cross post, a second post is created, posted in the community you choose to cross-post in, but the new post's rewards are denied as you can't monetize the same content twice on Hive. But those who come across your post, can still click the link that takes them to the original post, that can be upvoted. I hope it's more clear now.