DiagROM is a diagnostic tool for the amiga computers. And it is to be used on defective machines, trying to help no-boot situations etc.
You simply replace the Kickstart roms in the Amiga with the DiagROM. and when poweron. your Amiga will start a diagnostic software. It will try to dump info on the serialport (and actual menus etc aswell) while also trying on the screen.
One important thing is that the software relies on as little hardware as possible. so no timers, no IRQ etc. so you can start up the machine on a very broke machine and from that test different things. and hopefully help finding hardwareissues.