The Disclosed Quantity (DQ) option allows you to disclose only a part of your order quantity, to the market makers. In simple terms, DQ hides your actual quantity from the market and enables you to trade in a discreet manner.
For instance, Mr. Siddhu, a client of Fyers, wishes to place 10,000 Qty (Intraday) in ICICI Bank Ltd. If Mr. Siddhu places the order, the entire 10k Qty will be shown in the market under market depth. However, he can enter 2,000 Qty in the disclosed quantity and only 2,000 Qty will show in the market depth. This way Mr. Siddhu can trade discreetly without any concerns.