This means that OCD will be lending some of their voting power to OCA -- basically.

In exchange for OCA being a sub-community of OCD, and for users promoting the POSH initiative, by sharing their posts on Twitter using the Steempeak URL.

The only thing I don't know how to do yet is crosspost, or why I would cross post.

Is that from one community to another?

So you could crosspost from OCA to OCD?