Electronic energy meter - What does an electronic energy meter measure ?

I would like to refer to the knowledge of those "in the know" , what does an electronic power meter, such as one installed for example in a house or flat, actually measure. I mean, is it active or reactive power. I am asking in the context of the advertisements for the "wonderful" American electricity-saving devices. It seems to me that they sell simple capacitors that change the cos fi between current and voltage when the mains load is a motor (washing machine, mixer, lawnmower,...)
Does anyone know anything about these wonders and have up-to-date knowledge of these meters?

There was a forum about it, unfortunately I forget what the device is called here.
Active.

https://www.elektroda.pl/rtvforum/topic2960884.html

In private premises, active energy is currently measured and paid for. Reactive energy is not subject to payment/for the time being.