Why A prescaler is a fast digital divider placed ahead of the counter's gate circuitry. It divides the incoming signal by a fixed ratio, typically 10 or 100, so a signal well above the counter's maximum input frequency comes down into a range the counter can resolve. You then multiply the displayed reading by the prescaler ratio to get the true frequency (or the counter does it for you by stretching the gate time).
Watch out The choice about multiplying the signal has it backwards: multiplying would push the frequency even further beyond a low-frequency counter's limit. Amplification is the job of the counter's input preamp, not a prescaler.
Pre-SCALE means divide down. Prescaler = divide by 10 or 100 to fit a fast signal into a slow counter.