Watch the Slash Chords online guitar lesson by Rich Maloof from Guitar 201: Breakthroughs
Slash chords tell you which note should be played as the bottom note, or bass note, of the chord. The chord name comes before the slash and the bass note comes after the slash.
Read slash chords like D/C as D over C. That is, it’s a D chord with a C in the bass.
Am/G = A minor over G. A minor with a G in the bass.
D/F# = D over F#. D major with an F# in the bass.
Sometimes you have to change your fingering to catch a bass note, and sometimes the slash-chord fingering is even easier than the normal chord would be. Check the Charts here for several common slash-chord shapes and the names of bass notes along the bottom strings.