Captions and Titles are fields that are already available in images in the Media Library, as you can see in the image below. When you upload an image you can add, amongst other things, a Caption and / or a Title. The media based Elements with captions then pull that information directly from the images to display using the new Captions styles.
For images that don’t have captions and titles embedded in the file, you can add them (or override them) in the Element itself. There is one main difference however, between the Image Element, and the Image Carousel and Gallery Elements. With the Image Element, you can add the individual caption and title text on the Captions tab of the Element as it is a single image Element, but with the Gallery Element and the Image Carousel Element, these Image Title and Image Caption options are found by editing the individual Element items, found under the Children tab.