Reversi is a game which requires the strategic placement of checkers to control the game board and ultimately possess more of it than your opponent.
Although many people have played Reversi as a child, it is actually a very tactical game which requires much forethought.
The corner squares are the most important squares to own, as they will control the board throughout the game.
You should try to place your pieces on these squares as early on as possible.
Of second importance are the squares at the outer edge of the board, as they will control the lines going across.
The best move is not always the one that takes more of your opponents pieces.
A move that takes fewer pieces, but is of strategic importance in regard to position on the board or in regard to future moves, might be tactically better.