Click a square, you get a number.That number is the number of how many mines are surrounding it. If you find the mine, you can open "unopened" squares around it, opening more areas. The numbers on the board represent how many bombs are adjacent to a square.
Clicking with the left mouse key should reveal what is under the cover. Clicking with the right mouse key should place a flag to indicate the possibility of a mine. We mark cells with a flag to mark them as mines and so to avoid clicking on them with the left key.