the blue dot indicates the position where black just moved. and the 4 red crosses indicate all the legal moves available to white.
[ some reversi strategy/detail: when black just moved at c5, this kind of move is called "checker-boarding". ie , black has just "checkerboarded" white. the idea is that black has just "split-up" white's seeds "all over the place" . which is not good for white. so, in return, white has to regain control of the board. the correct/recommended move for white will be to play accordign to "flatness". and according to the <theory of flatness>, when "flatness" is achieved no matter what positioning of the seeds on the board, there is a distinct partition of the black n white seeds into distinct "localised groups". and when flatness exists anywhere on the board, that part is tricky to play..coz "flatness" is a double-edged sword... ]
after g6, black will carry on with h4, in a bid to maintain "flatness"
the reason why white moves to g6 is because it wants to create a white seed at f4, which then can be used to move at d6. and why want to move to d6? coz its the only way left for white to let its seeds "stay in the centre"
actually this opening is considered a difficult opening for white to handle. from here till to midgame and till to endgame, its not easy for white to counter against the designs of black...coz in this particular opening, the advantage of "flatness" is more biased towards black.