To describe left hand muting in a sentence, it is basically muting out a sound by slightly lifting your fingers off of the frets you are holding and continuing to strike the strings. However, you don't completely lift them off, but you lift them so that they are gently resting on the strings. So if you instructed to mute a chord, you would play that chord and then lift your left hand fingers off of the frets to muffle the sound and carry on strumming the chord. Here is an example of this...

e|-----------------------------|
B|-----------------------------|
G|--9-x-9-x-7-x-7-x-5-x-5-x-4--|
D|--9-x-9-x-7 x-7-x-5-x-5-x-4--|
A|--7-x-7-x-5-x-5-x-3-x-3-x-2--|
E|-----------------------------|

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Now try to play this example which uses left hand muting...

e|-----------------------------|
B|-----------------------------|
G|----------3-3-----------6-6--|
D|--3-3-3-x-3-3-0-6-6-6-x-6-6--|
A|--3-3-3-x-1-1-0-6-6-6-x-4-4--|
E|--1-1-1-x-------4-4-4-x------|
Nirvana - Smells Like Teen Spirit

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