To play a CDI file on an original Dreamcast console, you must burn it to a high-quality CD-R.
Because CDI files have downsampled audio/video, you are playing an inferior version of the game compared to a GDI/CHD file. On emulators, always prefer GDI or CHD. Reserve CDI only for burning discs.
To create a CDI, "rippers" compress or remove non-essential data from the original GDI file: : Reducing the bitrate of audio or video files.
A is a DiscJuggler image format that became the industry standard for the Dreamcast scene. Because standard Dreamcast games were released on GD-ROMs —a 1.2GB format developed by Yamaha—they cannot be burned directly to standard 700MB CDs without modification.