I'm not the author of ZeroNet btw, just in case you think I am.

Each site has an area for user content. That is content not generated by the site owner. Users of the site that generate content (posts in a forum or comments on a blog for example) write their content into that area which is a directory for them based on the public key of their pseudo-anonymous identity. The user then signs the data with their private key and this gets sent to other site users through the seeding mechanism. All this is hidden by the API so the user of a site doesn't see the public/private key signing, etc.