How to read the field
Every number tells you how many mines touch that square horizontally, vertically, or diagonally. A blank opened area has no adjacent mines and expands automatically.
Two basic deductions
If a clue touches exactly as many unopened squares as its number, all of those squares are mines. If its required mines are already flagged, every other unopened neighbor is safe.
Mobile flagging
Right-click marks a flag on desktop. On touch devices, switch on Flag mode before tapping suspected mines. The board is generated locally for every new round.