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Explains itself really

Perhaps not the part where you win by marking all bombs? (And only the bombs)
You would have figured it out. We have faith in you.

The number indicates how many bombs can be "seen" from this spot, in eight (8) directions.

You can not see through a bomb, e.g. the "1" in the lower right corner only sees the first one to its left.

You may use right-click to mark and unmark where available.