To expand on what Spaw said with an example, suppose you have your A string tuned to 220 hertz (vibrations per second). Your E string is tuned slightly flat, and when fretted at the fifth fret and plucked, produces a tone of only 215 hertz. The vibrations of the two string moving in and out of phase are what creates the beats. The difference between the two (5 hertz) is what causes the frequency of the beat. (as an aside, two different pure tones played simultaneously can create a "ghost note" - the new tone coming from the differnce in the played tones) The farther the strings are out of tune, the higher the difference in hertz of the ghost note, and the faster the beat. If you have a instrument with good sustain, you can note a string that is in tune and the one you want to tune, listen for the beats, and adjust the tuning until the beats disappear. At that point the two strings should be perfectly in tune with each other.