Complex sentence: Most stars above about 40 solar masses and having solar or less metallicity collapse directly into a black hole-the exception being for very massive but low metallicity stars, which may blow themselves apart upon collapse in a pair-instability supernova, leaving no remnant.
Split into simpler sentences: Most stars above about 40 solar masses and having solar or less metallicity collapse directly into a black hole. Some very massive but low metallicity stars become unstable due to pair production and may blow themselves apart upon collapse in a pair-instability supernova, leaving no remnant.