it's the amount of power available to each of the 3 voltage rails the pc uses. 3.3v, 5v and 12v. the 12v is the most important as most of the pc uses that power. so your psu is actually very weak since it only has 16 12v amps which is 16*12= 192w of power.
so essentially your psu is not 450w but rather only about 200w. many years ago the pc used the 3.3v and 5v rails a lot more so they were more balanced but as the years go by more and more goes to the 12v rail.
your psu is new enough that it should have more 12v watts, which tells me this is a very poor quality power supply and that it should be replaced.