The aim of Tapslide is simple: match the tile pattern seen on the opposite side of the playfield.
Actually succeeding to any degree of skill with Tapslide is another thing entirely.
But it's only tough because most people's sense of spatial awareness is underdeveloped, malnourished, and in need of some TLC. So by playing Tapslide, you'll not only get better at the game but, in theory, at a whole bunch of other daily tasks as well.
People say games don't teach you anything? Pah! Point them at the video below.