Assigning metadata to images is, at the end of the day, what Capturebot is all about. In order to do this you'll need to import images.
By default Capturebot will import any image, movie, audio file, or folder. You can do this by dragging files into the images area on the right or by using the "Import Images" menu command.
Dragging and dropping a folder of images will import the files contained in that folder. If you'd like to import files in subfolders as well use the "Import Images" command and ensure "Import Subdirectories" is enabled.
Capturebot can either embed metadata in the files themselves or write netadata as XMP sidecar files. You can choose which option to use in the preferences. When writing as a sidecar, the type of file doesn't matter, as Capturebot isn't modified the file itself. However, not all files can be written to directly.
If you're embedding metadata directly into files you're limited by the formats Exiftool can write to. If you've selected any files that can't be written to a warning icon will appear next to the image count. Clicking on this will show a popup displaying how many images can't be written to