The galvanic isolator prevents electrolytic corrosion. It blocks low voltage DC currents from entering your boat through the shore power ground wire. These currents can cause corrosion to the underwater metals of your boat, such as the hull, propeller, shaft, etc.
The galvanic isolator consists internally of two diodes that are connected anti-parallel. When connected in this way, the diodes allow current in both directions but only above a certain threshold voltage. The voltage at which the diodes conduct is approximately 1.4 VDC.

The isolator is installed directly behind the 230V connection of your boat. The forward voltage from the galvanic isolator is higher than the potential difference between the metals. As a result, this voltage will not allow conduction, and as such, the galvanic isolator will prevent any electrolytic current. However, if there is an error (higher) voltage in the AC circuit, the diodes will allow the current to pass and the residual current device will break the circuit.