Codex: Civilians gives you the ability to field civilians who take an active role in your battles. Civilians are aligned with one specific side in the conflict. This can be determined randomly or explicitly as part of a narrative for the game. For example, you might have the Dark Angels defending an Imperial colony against the predations of a newly uncovered Necron tomb. Or you could have a human genestealer cult aligned with a Tyranid assault force intent on consuming an Imperial stronghold. Regardless, civilians become part of the action, both offensively and as part of the mission objectives.
You can download the PDF for Codex: Civilians here.