What is a Slash Chord?
A slash chord is a chord which indicates emphasis of a bass note other than the root of the chord. When a chord is played it is typically assumed the bass will emphasize the root of the chord. Occasionally a different note is preferred and results in a chord with an alternate bass note.
Slash Chord Notation
Slash chords are notated with a chord's standard chord symbol, followed by a forward slash, followed by the alternate bass note. For example, G/B or D7/F# are slash chords.
When discussing slash chords musicians will typically say, "Play G slash B," or "play G over B," or "play G with a B in the bass." These descriptions all mean the same thing.
The Purpose of Alternate Bass Notes
The first question most students ask about slash chords is "Why would you need a different bass note?" The most common reason is the music's composer wants to stress a particular bassline in a chord progression.
An often seen chord progression is: G - G/B - C. In this progression the G chord is being played by the entire group on the first chord G. But, on the G/B chord the band continues playing the G chord while the bass player is to stress the note B which makes a smooth, chromatic transition to the root of the C chord. So, the bass would emphasize the notes G, B, then C on each of these 3 chords.
Another example progression might be: Am - Am(maj7)/G# - Am7/G - D/F# - F. This is a very common scenario where the alternate bass notes create a smooth, descending chromatic bassline -- A, G#, G, F#, F.
Though it's not required, very often the alternate bass note of a slash chord is another note from within the chord. In the previous example of G/B the note B is the third of the G major triad (G, B, D). Or, in Am7/G the note G is the 7th of the A minor seventh chord (Am7 = A, C, E, G).
How Bassists Address Slash Chords
Many times the alternate bass note is the only note you'll play on a slash chord though it doesn't have to be. Generally you'd at least want to stress the alternate bass note on the beat when the slash chord occurs. After that, if it is musically appropriate to play other notes you could address the chord like you normally would.
Listen to and learn to play some of the example songs to hear some slash chords in action.