Captions and metadata can be displayed in Foliobook Galleries.
Metadata can be imported from the IPTC fields for captions, descriptions and copyright statement embedded in the files Foliobook imports. Alternatively metadata can be added inside the app in edit mode in a gallery.
There are two metadata display styles available, the first one uses a simple bar at the bottom of the screen and has the caption justified on the left and the copyright statement of the image on the right. The transparency of the background is the only customisable style at the moment, and the caption display can be switched off if its not required.
Bottom metadata/text style 
To add text to images you can either add this as IPTC metadata in a tool like Lightroom, Photoshop, PhotoMechanic, Capture One etc.
Adding text in IPTC helps protect intellectual property by associating copyright data with the images.