Click a square, you get a number. That number is the number of how many mines are surrounding it, including squares diagonal from the number.

If you find the mine, right click to place a “flag” to mark that it is a mine.

Once you find a mine, you can open "unopened" squares around it, opening more areas.

Clicking both the left and right mouse buttons simultaneously over a number that already has it's mine(s)  found. This will open more blocks around it.