Electrical current is carried by electrons in a wire. Unless the wire is superconducting, the electrons encounter resistance, bumping into each other as well as the cable or wire through which they’re traveling. When that happens, some of the electricity is given off as heat and wasted, a phenomenon known as line loss. But increasing the voltage along the line allows you to decrease the current (and hence the line loss) without sacrificing how much power you’re transporting.
There’s a simple equation for calculating power loss: the current squared multiplied by the resistance. So if you lower the current, you lower the resistance. And if you lower the current while raising the voltage, you can keep the resistance low without sacrificing the amount of power you’re transmitting.