Consider this equation:

Power = Voltage x Current

Let's say you are transmitting a power of 100W with 5V and 20A(5x20=100W).

If you increase the voltage to 50V, how much current will you need to keep the power at 100W? Ans: 2A. Similarly, if you increased the voltage to 1000V, the current will be 0.1A to keep the total power at 100W(1000x0.1=100W). This is why we say 'high voltage means less current(amperage) for the same power'

If you understood the above, you can also see why we prefer high voltage and low current to transmit electricity. The wire through which the electricity passes only 'feels' the current passing through it, not the voltage. Hence, a higher current means more power losses during transmission. So we try to reduce the amperage as much as possible to avoid power losses during transmission.

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