A servo mechanism is basically an actuator (such as a motor) with a closed loop feedback system. What this means is that the servo controller can "know" the absolute position of the actuator at any time. In hobby electronics, one often encounters small servomotors whose position can be controlled by sending a control pulse of varying length (PWM).
Now let's say, for example, that you have a sonar distance sensor attached to a servo on the front of your robot, and you want the robot to turn the sensor to the right to see if there is a clear path. One could use a microcontroller to send a pulse to the servo commanding it to turn 90? to the right. What's nice about this is that the control electronics in servo will make sure that it gets to that exact position and stay at that position, even if the sensor is bumped. Now of course you can do far more complicated things with this setup, such as having the servo constantly scanning back and forth so that the sonar can create a 2d distance map ahead of it, but this is just a basic example.
If you want to learn more about hobby servos, I suggest that you check out this article by Sparkfun: https://learn.sparkfun.com/tutorials/hobby-servo-tutorial
It is comprehensive and very easy to understand. The Society of Robots also has its own page on this too: http://www.societyofrobots.com/actuators_servos.shtml