Okay, back to symmetry, an axis of proper rotation (often just called a rotation axis, or a Cn axis) is a symmetry element found when you rotate an object by some fraction of 360 degrees and the result looks identical. E.g. if you rotated an equilateral triangle by 120 degrees, you would see the exact same triangle. This is called a C3 axis, since you are rotating by 1/3 of 360. Since LEGO bricks are mostly squares and rectangles and fit together most easily with 90 degree angles, it is pretty common to see LEGO creations with C4 or C2 axes (of course these often require that you ignore the word LEGO written on the studs or other minor imperfections that break symmetry).
However, with some clever building, you can come up with LEGO creations with other rotational axes. These creations have C3, C5, C12 and C16 axes, respectively.