A double knock out (dako) is a European vanilla with two American barriers, one of which is set above the underlying asset and one of which is set below it. That means that one of the barriers is a regular barrier (i.e., it is hit as the option moves out of the money) and one is a reverse barrier (i.e., it is hit as the option moves in to the money).
If the underlying asset hits either of the barriers during the option's lifetime the option is terminated. If neither of the barriers is hit by the expiry date, the payout is that of the underlying vanilla option.