The galvanic isolator prevents electrolytic corrosion. It blocks low voltage DC currents that enter your boat via the shore power earth wire. These currents can cause corrosion to the boat’s underwater metals, like the hull, propeller, shaft and so on.
The galvanic isolator consists internally of two diodes which are connected in anti-parallel fashion. When they are connected in this way, the diodes allow current in both directions but only above a certain threshold voltage. The voltage at which diodes conduct is about 1.4 Vdc. 

The isolator is installed directly behind your boat's 230V connection. The forward voltage from the galvanic isolator is higher than the potential difference between 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 a (higher) error voltage in the AC circuit, the diodes will allow current through and the residual-current device will break the circuit.