Draw some shapes on a piece of paper: a square, a triangle, a circle, etc. Just lay them down randomly on the paper. Now, looking at the paper you will see, perhaps, that the triangle is at the bottom whereas the square is at the top of the page.
Walk around to the other side (or simply turn the paper), and now you'll find that the triangle is that the top while the square is at the bottom. Yet nothing about the relationships of the shapes to each other has changed.
The shapes on the page have some properties that are invariant with respect to rotations in my reference frame: the distance from the centre of the square to the centre of the circle, for instance, in invariant with respect to rotation. They also have properties that are reference frame dependent and are not invariant with respect to rotation: for instance their distance from the top or bottom of the page as defined by the page's position in relation to me.
That seems pretty obvious and even silly, but a very interesting thing about our universe is that the distance between two objects and the duration between two events is reference frame dependent, it is simply an arbitrary description of the underlying 4D reality, much like my description of the shapes on my page as nearer to the top or the bottom. There is still an underlying reality, but it is not made of distances and durations, it is made of spacetime separations.
