In the array to the left there are some hidden mines. You have to locate all these mines without hitting them, so they won't explode.
If you think that in a certain field a mine is hidden, just mark it with a small flag by clicking the field while holding down the
ALT key (depending of the browser and operating system, you may need to hold down the Shift key, the Apple key or the Control key
instead). If you think a certain field is free of any mines, just click on it without any keys pressed. This will
uncover this field and the number of mines is shown, which are hidden in the surrounding 8 fields. This way you
get hints about where other mines are hidden. You have won when you've found all mines. You loose as soon as you try to uncover a
field where a mine is hidden.