Reversi
Flank a line of discs and they all turn over. Corners decide everything.
Flank a line of discs and they all turn over. Corners decide everything.
Reversi is the game where being ahead is usually bad. Every move must trap at least one enemy disc between two of yours, and everything trapped flips to your colour — which means the board can turn inside out in a single move, and frequently does on the last one.
The whole strategy comes down to two ideas that beginners get exactly backwards: discs don't matter until the end, and corners are worth more than everything else combined. A corner can never be flipped, and it anchors an edge that also can't be flipped. Take a corner and you own that quarter of the board forever.
Because you're counting discs, and discs mean nothing until the last move. Every disc you own in the middle is a disc your opponent can flip. Strong Reversi players deliberately keep FEW discs in the midgame — it limits the opponent's options and keeps your own moves flexible. The flips come at the end, all at once.
Mobility is simply how many legal moves you have. Take it away from your opponent and they're forced into terrible moves — often giving you a corner. The engine here scores mobility heavily, which is why it will sometimes make a move that appears to gain nothing: it's shrinking your options.
The rules are essentially those of the game sold under that name — but 'Othello' is a registered trademark, and Reversi is the older, public-domain name for the game, dating to the 1880s. So Reversi is what we call it, and Reversi is what it is.