Your current -especially in an arduino circuit will have fluctuations, it will not be DC. It will have a very complicated spectrum. The power supply I just mentioned has a chart showing the real part of the impedance out to several MHz. If your device draws current at a frequency where the impedance is high, you can have voltage variations.
Output resistance refers to the classic Thevenin equivalent where you have a voltage source and resistor to describe your power supply black box. If the resistance is high, you don't have a very good power supply because the voltage will change a lot with the applied load. So, you use feedback and remote voltage sensing to lower the output impedance as much as possible to get good load regulation.
A good power supply will have a very low output resistance.
But output resistance has nothing at all to do with how much current it can supply. Any decent power supply will limit the current internally long before it would be limited by the output resistance. For example, the 7805 regulator limits the current to 1.5 A, but has an output resistance of .017 ohms. If the current was actually limited by the output resistance, it would be able to supply 5V/.017 = 300A!