To elaborate a little further, you place the palm against the strings, flat, with the fingers pointed upward, and play with the thumb, using your palm to muffle the strings below, and if it is a sequence of notes, then to return to the string played and muffle it as you get ready to play the next note. It is not an abrupt muffle, as far as I have seen. It could also be used with the fourth finger, I think, to signify muffling the strings above the bass note.