A series is a group of photos with a common theme.  For example, I shoot a lot of models, and I create a series for each one.  The series name is usually the model's first name.  (For an example, visit www.canstockphoto.com/series/carol-18313).  That way, if a potential buyer looks at a photo that comes up in a search, the series link will be present as he/she is viewing the photo.  By clicking on the series link, the buyer can view all of the photos of that particular model.  You could do the same with, say, photos of construction, or underwater shots, or whatever.

At the bottom of the "My Portfolio" page, you'll see a "Series Wizard" link, which will take you to the series editor where you'll give your series a name and a brief description, and select photos to add to the series.  You can then add photos to the series whenever you wish.  For example, if I upload a new batch of photos of a particular model, I can add them to the corresponding series.

"A series is any batch of photographs that, when uploaded to Dreamstime, will be rejected as too similar".   

Yes! +1