A silver pool is built out of the silver sinks (repair, transmutation, quality improvements) which remove some excessive gold from the market. In plain terms, this means a discount on anything that requires silver.
The image to the right taken from an in-game screenshot is an example of a global discount being applied to an island upgrade. In this case, the upgrade cost of 7500 silver has been reduced to 4350 silver due to a global discount of 42%, so a gold price of 630: (630/1500*100=42%)
Global discount % works as it follows:
|Gold price||% of discount|