Data is trying to formulate a battle plan for Commander Riker, but he’s assuming that Commander Riker is rational and knows that Data has analyzed his move. Data takes it a step further and hypothesizes that Commander Riker knows that Data knows that the commander has a battle plan. Full information is a tough assumption about rationality, but bounded rationality lets us assume that people have limitations but still respond to incentives in a predictable way. While perhaps a human failure, most of society does not operate on the same level as Data.
Thanks to Peter Nencka for the clip suggestion!