Palm muting is a very common rhythmic effect in rock and metal, but not exclusively used in those styles of music. It is a means of striking chords and notes whilst damping the strings with your strumming hand as you play the strings.
Don't confuse palm muting with muting. Palm muting is a way of muting strings with your strumming hand, and muting is a way of muting strings with your other hand on the fretboard. Although both techniques are commonly used interchangeably, here we deal with palm muting exclusively. Muting is explored in a separate Core Skills section.