What is it?
This technique imparts a percussive, muffled/chunky sound to the string's tone and is desiderable as an effect in many styles of music as rock, metal, r&b, country, pop and fusion, to name a few.
Pal muting is indicated by the abbreviation "P.M." appearing above the tablature.
How to play it
Palm muting is a technique whereby you rest the heel of your pick-hand palm on the string as you pick. The palm of the picking hand should rest lightly on the bridge with the underside of the forearm resting on the front of the guitar body.
It is important to rest directly on the bridge, not in front of it or behind it. If you are in front of the bridge (on the side closer to the neck), you will be over-muting, your sound will be too muffled and your pitches may be affected. If you are behind the bridge, you will be under-muting, your sound will not be muffled enough, and may not be muffled at all.
In the example below palm muting is used on single notes.
In the example below palm muting is used on chords.
Here palm muting is used only on the 6th string open, this is an example of rythm with continous bass.