In the first few moments after the big bang, pressure and temperature were extremely great. Under these conditions, irregularities in the structure of the universe may have resulted in the compression of matter to densities large enough to create a black hole. It might be possible to artificially produce compress matter to such densities, but no natural phenomena after the very early stages of the universe could create such compression. Therefore the term primordial.
Some astronomers have conjectured that primordial black holes could help account for the missing mass of the universe.
Interestingly, because of the nature of their formation, primordial black holes are free from the constraint that their mass be more than about thrice the mass of the sun. All black holes are believed to emit Hawking radiation at a rate inversely related (proportional?) to their mass. Further, this emission decreases their mass, due to Einstein's mass-energy equivalence. Therefore, black holes with very small mass would "evaporate" in a massive burst of radiation equivalent to the explosion of millions of H-bombs. While regular black holes cannot lose all their mass within the lifetime of the universe (they would take about 1060 years to do so!), primordial black holes can. It has been calculated that primordial black holes that were created with a mass of about 1012 kg would have a life time about equal to the age of the universe (which means that we should be able to observe some of these explosions). Indeed, the evaporation of primordial black holes has been suggested as one possible explanation for the phenomenon of gamma ray bursts.