They mean that, in the squares touching THAT square (top/bottom, sides, & diagonal) there are that many bombs that you need to mark. You need to use the squares in relation to each other to figure which ones are safe to click on, so you can mark more mines. Hope this helps <3
The Minesweeper numbers indicate the several mines that are next to that square. This applies vertically, horizontally and diagonally. For instance, number 4 in a square will mean that there are 4 bombs that are adjacent to that tile.