A short, figurative answer from the electronical point of view:
A power supply (an an analogy you can view it as a water pump) as used by 3D printers is usually supplying a fixed voltage (a constant pressure going into your pipe system), in your case 24V.
The given amperage/current (the amount of water that actually flows) that is actually utilized at a given point in time is determined by whatever you hook up to your power supply (the system of tubes or pipes - imagine a valve like your tap/faucet). Now the amperage rating of your power supply gives how much current you can run through your electrical system (as is the flow of your tap/faucet by the pump if you had a limitless big, imigainative one).
The power that your supply can deliver is the product of voltage and amperage: P(power) = U(voltage)*I(current).
What you need to make sure is that a) you deliver the correct voltage, because this is what your circuitry needs to be specifically designed for (image the pressure of your pump being too low or to high, - you either won't recieve any water in the second floor, or your tubing can't stand the pressure) and b) that you can supply at least the needed power (otherwise you get a problem once you open up all the taps/faucets in your house, because they don't supply as much water as demanded). If your power supply can give more current, that's fine, it might just not be used. And as mentioned by Thetravellingfool already, keep a certain plus for losses and as a reserve, because no pump likes to run constantly at it's limits either ;)