Cross-hatched cells are hidden. You can place your mark there, but you won't see whether the opponent has already taken the cell.
If you play against the AI, the computer sees just as little as you do and makes its decisions under the same uncertainty.
Five in a row wins. After the game, all hidden cells are revealed.
Five in a Row in a partially observable environment
In regular five-in-a-row, both players see the whole board. But what happens if parts of the board are hidden?
1
Hidden cells. Cross-hatched cells are hidden. You can place your mark there, but you won't see what's underneath.
2
Blind moves. If you play on a hidden cell, you won't know whether the move succeeded. If the cell was already taken by your opponent, the move is lost.
3
Five in a row wins. Same rules as usual. Whoever places 5 in a row first wins.
This is called partial observability. The agent, that is, the one making the decisions, sees only part of the environment's state.