Think of it as volts=pressure, amps=flow.

If you supply too high a voltage, you'll damage your device; too low, and it probably won't work (but I don't know if it will cause damage).

For current, the power supply is rated for how much current it can draw safely (or stay within spec); if the current rating (amps) of the power supply is higher than the device, it's okay, because your device won't draw that much current. If it's too low, you're overloading the power supply, and either your power supply might fail, or your device might not be able to draw as much current as it needs to function (or charge).

So: the voltage should match; the power supply amp rating should be at least as high as the device rating. (At least that's my understanding; someone please correct me if I'm wrong.)