They are referring to voltage drop. Voltage is electrical pressure pushing current (amps) through a wire. Think of it like a water hose. Pressure pushes water through a hose. The walls of the hose resist the flow of water. As long as the hose isn’t too long, your pressure at the end is usable, but if the hose is too long, the pressure drops off along the way due to the resistance of the wall. Same thing happens to voltage in a wire.
When electricity flows through metal, the metal warms up. This is how kettles and other electrical heaters work.
Usually the cables supplying electricity to your house are thick and specially made to have Low resistance, meaning less electricity is used up as heat in these cables. But over Long distances there can indeed be some power loss as heat.
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