Many motherboards put one or more progress/status codes in a specific memory locations while they are running their diagnotics and these devices read those values and display them. The idea is that on a faulty unit that can't even get as far as driving a display you can use the supplied information (and sometimes the data supplied by the motherboard or computer manufacturer) to get an indication of which component has failed.
Under most circumstances, these failure codes refer to individual chips or sub-components (eg: keyboard controller failure or stuck interrupt line x) and so although you can perhaps see why a board has failed you are not likely to be able to fix the fault without some specialist board repairing and reworking tools.