The national grid is the network of cables, pylons and transformers that transfers electrical energy from power stations to homes and businesses. When current flows through a cable, the cable heats up and some electrical energy is wasted as thermal energy. The larger the current, the more energy is wasted this way.
The power transferred is the potential difference multiplied by the current. To transfer the same power, a higher potential difference means a smaller current. Doubling the potential difference means the current is halved. So the electricity is transmitted through the grid at very high voltage, which keeps the current low and reduces the heat lost in the transmission lines. This makes the transfer more efficient.
Very high voltages are dangerous and not suitable for houses. So in each locality the voltage is reduced to a much lower, safer value before it reaches homes, for domestic use. This makes it safe.