What is the meaning of the option of defect?

"Defect option" (Khiyar al-Ayb) is the right of one of the contracting parties to annul or uphold a contract when there is a defect in one of the two عوض (substitutes/exchanged items), of which the owner was unaware at the time of the contract. This defect either diminishes the value of the item or compromises its intended purpose. The reason for this option is the appearance of a defect that reduces the value of the sold item. The criterion for a defect is anything that reduces the value of the sold item, such as illness, or anything that commonly reduces the price in trade.