In the most basic terms, Herald is a choice-driven adventure game set during an alternate 19th century in which the West has united as a colonial superpower. As the entire story takes place aboard the merchant ship Herald its going to be up to you to ensure the peace between the passengers and crew by resolving their daily squabbles before they turn into something far worse. And if dealing with a bunch of angry people stuck in tight quarters wasn't hard enough as it is, the Herald is also a home to a great mystery which you will need to slowly uncover as the story progresses.
Since trying to explain all of this in a single paragraph is a bit difficult, allow me to instead show you the recently posted trailer. It does a rather good job of explaining what Herald and its story are all about. Have a look:
Before you dive into Herald it is important to mention that this is only the first part of the story - Books I and II. In total you can expect around four hours of gameplay, with books III and IV coming at a later date to further amplify that number. I'm not sure whether the next two episodes will be paid DLC or not, but given Herald's fairly low $10 price tag I think its safe to assume they will be.