The online DIS marker in your trade book indicates that Zerodha settled your sell trade using online delivery instructions instead of a physical Delivery Instruction Slip (DIS).
When you sell securities, Zerodha must debit the securities from your demat account to settle the trade. This requires delivery instructions, which you can provide either through a physical DIS or online via CDSL TPIN. Since all of Zerodha's processes are online, Zerodha cannot settle regular trades using physical DIS.
When you provide delivery instructions using online DIS consent, Zerodha tags this in their system for your information. All trades settled using this consent are marked with the online DIS tag in your trade book on Console.
You can also provide a Demat Debit and Pledge Instruction (DDPI). DDPI is a document that allows a broker to debit securities from your demat account and deliver them to the exchange. You do not have to enter the CDSL TPIN and OTP to sell shares once you submit the DDPI.