Every entity has a single entry field, and can have many posts. Posts are, fundamentally, just more entry fields for an entity.
Posts have a name, for example you want to add a Backstory post to a character. They can also have a location, for example if the backstory was in the town they grew up in.
Players who can edit an entity automatically get to edit their posts. If you don’t want to allow your players from editing an entity, but still add posts (for example their notes/clues about an NPC), you can give their role permission to edit the posts of the entity in the permissions subpage of the entity.