The board is a mine field, where some fields have mines under them. The goal is to find all mines, more precisely to open all fields which don't contain a mine.

Left click: opens a field. If it has a mine, then it explodes, and the game is over. If the field is empty, then it is opened, and a number is revealed that tells how many of the adjacent 8 fields contain a mine. The number 0 is omitted, but then all adjacent fields are also opened automatically.

Right click: marks a field for having a mine. The top left counter shows the number of remaining mines. The mark can be cleared by right-clicking on it again (except when question marks are turned on in the Settings, because then right click cycles through the sequence "mark → question mark → clear"). Marking mines isn't required, it only aids solving. In fact, the game doesn't even tell if the marks are correct or not.

Left+right click (or middle click): opens adjacent fields. If the field is already opened, and exactly that many adjacent fields are marked than the number of mines shown on the field, then this click opens the rest of the adjacent fields. This can speed up solving, but if one of the marks is wrong, then the unmarked mines explode, and the game is over.