When you are emulating a Dreamcast ROM, you need to have BIOS files in order to play. BIOS files are proprietary information needed to "jump start" the emulator and because of legal reasons, it has been excluded from the majority of emulators. However, you can still download it from here and load it in your emulator to start playing ROMs. Links are below followed by some general instructions on how to get started.
Note: Every dreamcast emulator is slightly different and requires a different BIOS file (so annoying). I've included all of them in one zip, you may need to tinker around to get it to work. I setup folders to help you out. These files will work for Chankast, Reicast, DEMUL and many others. Even so, try to find general getting started guides because it can be frustrating getting these to work.
Dreamcast BIOS (All, World) - PC, iOS
dreamcast-bios.zip - 6 MB
Dreamcast BIOS - Android [reicast]
dreamcast_bios.zip - 1 MB
Step 1. Download
Download the BIOS file above onto your PC or Android Device.
Step 2. Placement
If using Android
Extract all the files into into a folder called /dreamcast/ on your internal or sd card. Should end up with /dreamcast/data/bios-files-here.bin
If using PC
Place the BIOS files in the same directory your ROMs or Emulator is in (see emulator for specifics). Some emulators want you to extract, but with DEMUL leave them in the ZIP.
Step 3. Open Emulator
Open your emulator and if required, specify or load the BIOS file if necessary.
That's it! If you have any additional questions feel free to contact me.