Watch the Choking the... Chord online guitar lesson by Angus Clark from Hard Rock Survival Guide: Rhythm

Silence is half of rhythm playing. This is a simple concept that addresses creating negative space. Big chords need to be choked in order to get them to stop ringing. There are a lot of things that can go wrong when you do this. Your right hand can cause a scratching noise against the strings. Your left hand can cause a woof. Depending on your type of guitar and your level of gain and proximity to the amp the action required to get the chord to mute will be different. Let your ears be your guide.

Chukka is an imitative (or onomatopoeic) word for the sound of a down-up stroke on the muted strings. The muting is primarily done with the left hand, but you may need to mute with your right as well. You will see that at various frets the string will sound a harmonic or a pitch, so be sure there is more than one finger touching each string, and then if it's still causing a problem you'll need to bring your right hand palm in contact with the string as well. I've organized these muted figures using the following imitative words. This may seem simplistic, but this is often the fastest and easiest way to communicate what you want to other band members:
Chuk - muted downstroke
Chukka - muted down-up stroke (eighths or sixteenths)
Ga-Chukka - muted up-down-up stroke (usually sixteenths, think "Hymn 43" by Jethro Tull)