A voltage follower (also called a unity-gain amplifier, a buffer amplifier, and an isolation amplifier) is a op-amp circuit which has a voltage gain of 1.

This means that the op amp does not provide any amplification to the signal. The reason it is called a voltage follower is because the output voltage directly follows the input voltage, meaning the output voltage is the same as the input voltage. Thus, for example, if 10V goes into the op amp as input, 10V comes out as output. A voltage follower acts as a buffer, providing no amplification or attenuation to the signal. 

              An op amp circuit is a circuit with a very high input impedance. This high input impedance is the reason voltage followers are used. This will now be explained.

               When a circuit has a very high input impedance, very little current is drawn from the circuit. If you know ohms law, you know that current, I=V/R. Thus, the greater the resistance, the less current is drawn from a power source. Thus, the power of the circuit isnt affected when current is feeding a high impedance load.

Lets look at both illustrations below:

The below circuit is a circuit in which a power source feeds a low-impedance load. 

In this circuit above, the load demands and draws a huge amount of current, because the load is low impedance. According to ohms law, again, current, I=V/R. If a load has very low resistance, it draws huge amounts of current. This causes huge amounts of power to be drawn by the power source and, because of this, causes high disturbances and use of the power source powering the load.

Now lets look at the circuit below, connected to an op-amp voltage follower:

This circuit above now draws very little current from the power source above. Because the op amp has such high impedance, it draw very little current. And because an op amp that has no feedback resistors gives the same output, the circuit outputs the same signal that is fed in.

This is the reason voltage followers are used. They draw very little current, not disturbing the original circuit, and give the same voltage signal as output. They act as isolation buffers, isolating a circuit so that the power of the circuit is disturbed very little.

Voltage followers are important to buffer or isolate a low impedance load from a voltage source. This means that rather than connect a relatively low value of load resistance across the terminals of the power source, the op amp can be used to eliminate any loading that might occur. Thus, the power source will not be loaded down. The circuit acts as an ideal voltage source with nearly zero internal impedance, since it barely uses any current, yet outputs the full voltage.