RO - Reverse Osmosis
Osmosis - The diffusion of water along the concentration gradient through a semi permeable membrane.
Basically, reverse osmosis is putting impure water under pressure, causing it to pass through a semi-permeable membrane against the concentration gradient, causing the water on the outside of the membrane to be very very pure, very very soft and ph 7. also very low on phosphates and nitrates.
if parrots like ph7 hardness near 0, give them RO. If they like ph6 hardness near 0, filter normal water over peat. If they like ph8 high hardness tap water will usually suffice.
If u post back and say what condition they like (ihave never had them so dont know), we can advise on how to make those.