Non-isolated power supplies for guitar pedals, are in essence, one single power source that a specific number of pedals can draw current from. These are usually, a single source with a daisy chain cable. This will work fine in many instances, and is usually an accepted low cost solution in non critical applications.
However, the more pedals drawing from the non-isolated supply will increase the “capacitive load” on the supply. When this happens, depending on the pedals used, unwanted noise can be amplified in some pedals. Other factors such as the conditions of the venue that you are playing at, florescent lights, bad wiring etc., can ad havoc as well.
With an isolated power supply, each output is isolated from the others, it will lower the capacitance and quiet noisy pedals while shielding from interference of other sources. This is a lesson usually learned on the road when the guitar player is doing a live performance and has a bad experience with a non isolated PS, many of us have a story like this that ends with buying a new Isolated supply. While the isolated power supplies tend to be a more substantial investment, they offer more peace of mind and more reliable service. Some, like the ModTone power Plant, offer both DC and AC current for optimal versatility.