Definition:
Two circuits are galvanically isolated from each other when there is no path for current to flow from one circuit to the other. As a rule, this refers to direct current or alternating current with a low frequency. More on that below.
This is particularly interesting when you still have energy or a Signal from one circuit to another. So it's about creating a kind of coupling between two circuits that blocks direct current while allowing a useful signal to pass.
There are various components that can cause galvanic isolation. The most important thing is that transformer or transmitter. The two terms actually denote the same device, and in other languages, such as B. English, there is only one term. In German one says transformer when it comes to the transmission of energy and Übertrager when one wants to transmit a signal. The coupling mechanism here is via a magnetic field. The circuits are galvanically isolated because the primary winding and the secondary winding of the transformer are not connected to each other. (By the way, there are also so-called autotransformers in which the primary and secondary windings are connected. With these, no galvanic isolation is possible).
Other methods of galvanic isolation are e.g. B. Optocouplers (here the coupling is via light), capacitors (coupling through an electric field), acoustic couplers (mechanical coupling), or radio coupling (coupling via electromagnetic waves).
Galvanic isolation can e.g. B. may be necessary for security reasons. A mains transformer provides galvanic isolation between the power supply and the electronics inside a device. This will prevent you from getting an electric shock by touching a terminal.
Another purpose of galvanic isolation is to prevent interference currents (e.g. ground loops). So contains z. B. a sheath current filter a small transformer that passes the radio frequencies but blocks DC and the low hum frequencies.
A similar purpose is served by a NF-Transformer that causes galvanic isolation on an audio line and can thus also prevent ground loops. It should be noted here that their transmission bandwidth is limited.
So e.g. B. when used on subwoofers.
With digital signal transmission (e.g. SPDIF or Ethernet), the signal frequencies are much higher than with analog audio, which means that the transformers used can be much smaller and cheaper. Transformers are therefore more common in SPDIF than in analog audio. With Ethernet, transformers are mandatory. Optical SPDIF (Toslink) of course also provides galvanic isolation (it's sort of a distance optocoupler).