The outside faces of the cubies have stickers of 6 different colors, one for each of the 6 faces of the main cube. As you rotate the slices, the stickers become shuffled and the faces of the main cube become a jumble of colors. It looks like a simple puzzle, but as you move further away from the starting position, you begin to realise, as one writer put it, that you are like a small child who has let go of a helium balloon only to see it move more and more beyond reach or hope of return.
Mathematicians calculate that a 3x3x3 cube can be shuffled into 43,252,003,274,489,856,000 different patterns, yet they conjecture that all positions can be solved in 20 moves or less. The method that can do that (as yet undiscovered) is called God's Algorithm. Practical methods can take more than 100 moves. As there are 12 face-move possibilities at each step, the chances of solving the cube by trial and error are rather remote. Some cubes have 9-part pictures on the faces and that makes them even more complex, because you then have to get the orientations of the stickers correct as well as the positions.