a PSU has a number of "rails"
each is just a channel which power goes through.
every standard PSU should have at least one +3.3V, one +5V and 1 +12V 'rail'
a lot of people consider the +21V rail very important, because if it can give more power, they can usually put more things in, as most add-ons (fans, hard drives, lights) use 12V
basically, those 3 'rails' should at least be close to what it is rated (12V, 5V, 3.3V) and should not fluctuate (move around)
Amperes is a different measurement to voltage.
Voltage is the difference in charge between one end and the other. the electrons will try to balance it out, so a lot fo them move from the negative side to the positive side (electrons are negatively charged)
now Amperes is the actual amount of charge there is. the more amperes, the more electrons there are flowing through. if you have more amperes in a rail, more devices can be powered
so, a good PSU will deliver stable voltages, and a lot of amperes.
that PSU has 20A on the +3.3V rail, and 40A on the +5 rail, which is more than enough. since it is more so the motherboard that uses the +3.3V you see it is more stable with more amperes going through.
however the 12V rail only has 17A. it will probabbly do, but it isn't all that much.