A voltage follower is an op-amp circuit that has a voltage gain of one. It's also referred to as an isolation amplifier, a buffer amplifier and a unity-gain amplifier. The op-amp doesn't provide any amplification to the signal.
The voltage follower gets its name from the way the output voltage directly follows the input voltage of the mechanism, which means that the output voltage is the same as the input voltage. For example, if 20 volts go through the amp as input, 20 volts will come out as output. Because the voltage follower acts as a buffer, it provides no attenuation to the signal.