SlideShow will present your photos in a running slide show overlaying the titles you have entered using Apple's Photos app. SlideShow requires that you have at least one album set up in your Apple Photos library. You should use the Apple built-in Photos app in your Mac, iPad or iPhone to create your albums and to re-order the photos within each album. SlideShow only plays the albums you have already created. If you have also set up folders to organize your albums, SlideShow lets you navigate through the folders to select an album.
SlideShow's sole reason for being is to display titles on each photo. The advantage is that you can set your title in the photo's metadata without permanently overlaying text on the photo itself. (Metadata is information about your photo that is embedded in each photo's file -- and includes information about when and where you took the photo as well as information, like the title, that you can add.) You then have access to the unaltered photo but can also display a slideshow with the titles you have set.  
Unfortunately as of iOS 16 and MacOS 13 you can only view and edit the titles with the Photos app on your Mac or in iCloud Photos (i.e., your Photo Library accessed through a browser at icloud.com). The title field is accessible in the Info box for each photo (Command-I). iOS and iPadOS only allow you to view and edit photo captions, but these are not available to be read by any iOS app. Titles are available to be read by an app, but can only be edited on a Mac or in iCloud. Go figure. SlideShow will display the titles for each photo and optionally the date of the photo. SlideShow cannot display captions or keywords.
SlideShow will request permission to access your Photos library. You should grant access to all photos. SlideShow will not make any changes to your photos or your library, but requires access so it can display the photos in a slide show.