A lull turn happens when you roll doubles. So say you rolled double 6's, the rule book is telling you to put a ship/ships in the ocean that you rolled doubles in. In this example you would place your ships in 6.
If you were only rolling TWO white dice, then yes, you would place two ships in Ocean #6 (the Indian Ocean, if memory serves).
If you rolled THREE white dice, you would also place a ship where the other white die indicates, in addition to the two in the doubles Ocean.
You get to IGNORE the black di(c)e during a LULL turn, so there's a break, and place a Gemstone in the doubles Ocean and on the top of the Adventure deck (potential treasure!!).