The exercise is best understood if you imagine it as two exercises in one.

Let's say the first grid you see after starting the exercises is this one (we can imagine this as the first exercise):

This means you have to memorize the blue square inside the grid, so we memomrize it is the upper left hand corner.

Now select "Memorize", which will result in this grid to be shown (which can be seen as the second part):


This figure (i.e. the two orange coloured squares in the lower left and right corners of the grid) is vertically symmetric. This means the figure can be divided into two equal parts if you "cut" it vertically:

Or geomtrically spoken, it is mirrored on the y-axis (which is the vertical axis).

The next (four square) grid is the one from what we see as exercise part one:

We memorized that the upper left hand corner of the grid was coloured blue, so we select the applying square:

This is the description of one task within the exercise. On higher levels, the patterns are more complex.