Tap a tile, then tap one of the highlighted tiles it can reach. The tile slides in a straight line until it hits the first tile in its path. Nothing slides into open space — there has to be something to hit.
A tile with nothing at all in its row or column is stranded. Nothing can reach it and it can reach nothing, and since tiles only ever leave the board, that never gets better — a stranded tile means the board is lost. The stranding guard outlines any move that would create one, and marks a stranded tile in red if you make one anyway. Turn it off once you are reading the board without it.
The gold ring marks the final square. Clear the whole board with your last move landing on that ring for the bonus. Every board can be finished this way.
Every board is built from a game code, shown under the controls. The letter is the rule set and the digit is the difficulty, so the whole code reproduces a board exactly — paste one in and press Load to play the same deal someone else played. Restart replays the board you are on without changing the code.