Anti-fog bathroom mirrors have a heated resistor which, when activated, prevents the build-up of water vapor which is created when you bathe or shower or when you wash with hot water in your sink. The end result is that you will see a clear reflection, at all times.
In the modern literature one can find many different names for anti-fog mirrors, which essentially refer to the same product, such as bathroom mirrors with anti-fog film, anti-fog mirrors, anti-steam mirrors, etc. However, they all have the same effect.
Scientifically documenting the phenomenon of water vapor condensation, i.e. how it is created, we would say that condensation occurs on glass surfaces, when the hot water from your shower or sink turns into steam and then, after cooling down again, turns into a liquid.
Unfortunately, surfaces that are cooler than the surrounding air, such as mirrors, tend to fog up, which means their reflective properties are weakened.
In order for this not to happen, the temperature of the mirror surface must be closer to the temperature of the air, and this is exactly what anti-glare films do.
When the film resistance is on, a heating element heats up the mirror surface, meaning condensation is much less likely to form, resulting in a vapor-free mirror.