When you tap on a tile in the puzzle, it will move to an empty space in the board at the bottom. When there are 3 tiles of the same image in that area, these tiles disappear, leaving room for more tiles.
Because there is only space to accommodate 7 tiles at a time, you must be careful as to not tap on tiles at random. You should only tap on a tile, if you are sure that you can match 3 tiles with the same image. Otherwise, you’ll fill the board with a bunch of random tiles and you won’t be able to add more tiles once the space is filled.
When the board is full with 7 tiles, it is game over. So, focus on tile matching and enjoy the relaxing game.