Muted Strumming is a percussive effect on the guitar. Iit is usually something that will happen naturally when you learn how to play. There is a couple of ways you can do this style.
(this is the left hand for right handed players)
This is when you strum normally but lift the tension from the fretting (left hand). When you strum, the guitar will produce a "thud" or a muted sound from the strings. Be careful not to release your fingers all the way off the strings and produce an open string sounding.
This style of muted strumming can be done on both the down and up strum.
It doesn''t work well for open chords because you either have an open string sounding out or have to reshape your hand for a muted sound, then place it back to the chord shape again. This style of muted strumming is best kept for bar chords.