A slash chord is a guitar chord which uses an alternate bass note for the chord. Typically a chord is composed of the root note (or tonic) and the third and fifth. A slash chord substitutes/augments the root with another bass note, which can be just about anything. The purpose of this is to liven up the sound a bit, and to make it easier for one chord to lead to another by using a bass note that leads to another note better than the tonic.

A slash chord is notated as X/Y where X is the chord and Y is the alternate bass note. A fairly common example is a C/G. A C chord is played as x32010. If we substitute the tonic (a C) with a G, we get 332010. Note that in this case we do not actually get rid of the original tonic, we simply add a lower bass note, in this case, a G. In other situations, you may end up not playing the tonic note at all.