In the lowest regime, you have the worst possible case scenario, where the computer constantly experiences bit flips from every possible cause imaginable. It has so many problems with bit flips that it's basically unusable. For every level that you go up, you travel into a new 'error regime', where the frequency of bit flips is decreased by an order of magnitude. In the top regime, you have a computer that experiences bit flips as infrequently as possible given the known laws of physics.