Reversi (a.k.a. Othello) is a board-game based on a grid with eight rows and eight columns, played between you and the computer, by adding pieces with two colored sides.
Each player must place a piece in a position that there exists at least one straight (horizontal, vertical, or diagonal) line between the new piece and another piece of the same color, with one or more contiguous opposite pieces between them.
After placing the piece, the player turns over, i.e. captures, all pieces of the opponent player lying on a straight line between the new piece and any anchoring pieces with player color. All reversed pieces now show the player color. A valid move is one where at least one piece is reversed.
Black color starts first move. Players take alternate turns. If one player can not make a valid move, play passes back to the other player. When neither players can move, the game ends. The winner is the player that owns more pieces.
A very good way to improve your skills in Reversi is to look at real tournaments played by experts: 2019, 2018, 2017, 2016, 2015.
Here are a list of real games that you can navigate step-by-step to better understand it.
Normally a Reversi Game ends after 60 or less moves. The shortest game takes only 9 moves.
Here are a list of short Reversi games that you can navigate step-by-step to better understand it.