Duality means that the way we percieve is discordant to reality, not the same as what is in fact the case. The way it appears is out of harmony with the way it actually exists.
Language
Words of Wisdom
"As long as we consider any phenomenon of Samsara as good or attractive, we create bonds which will result in taking rebirth, through the process we call the “Twelve Interdependent Links of Origination”."