The goal of the game is to click on all of the squares without a bomb, without clicking on any square with a bomb. Each number represents exactly how many bombs are touching that square, including diagonally. When you logically narrow down which square has a bomb in it (E.g. because there is only one unclicked square touching a square with a 1), you put a flag on it, representing that there's a bomb underneath it. In this image, there is a square with a number 4. This square is touching three flagged squares, representing three bombs, and two squares that may or may not have a bomb. We know that there must be one more bomb touching the 4, but it's impossible to narrow down which square has the bomb. So you have to click on one of the squares, and there's a 50/50 chance that either you guess right, or you step on a bomb and lose the game. It's a fairly common scenario in an expert level game.

I got you.

Each number corresponds to the amount of adjacent bombs. The game randomly assigns bombs on a grid and it's the players who need to uncover everything except for the bombs. In this situation you can logic your way to knowing the three flagged squares are bombs. The square marked 4 has one more bomb next to it but there isn't enough information to know which of the remaining squares is the bomb. There are no extra lives in minesweeper. This 50/50 chance could lose the game