Does it look like some of the words are twisting clockwise and shrinking? You might see stripes in a spiral pattern, too. Try looking at other objects around the room.
All of your senses become dulled after any monotonous stimulus, which is actually a benefit: your brain notices what is changing. Close your eyes for a moment and listen carefully to the room you're in. Can you hear any faint humming noises? A refrigerator running? Heating or cooling systems? Traffic or birds outside? You probably weren't aware of those before you directed your attention to them. Now, while you were trying to pay close attention to sound sensations, were you aware of your feet at all? Can you feel your shoes, or the ground beneath you? You can feel that all the time, but you never usually notice unless you're in pain, or the sensation is important for some other reason. Okay, so now how is your tongue sitting in your mouth? Is there plenty of room for it in there, or is it squeezing your teeth and the roof of your mouth? How hungry are you? How often are you breathing? Could you use a trip to the bathroom?
These are all examples of things which you can choose to pay attention to, but which are normally filtered out of your consciousness. But even before the level of conscious thought, there are several layers of filtering that happen first--most of which you can't be aware of even if you try. If you walk into a room and notice a faint smell, after a while you won't smell it at all any more until you leave the room and come back, even if you're trying your best to notice it. There's actually a fairly simple physiological explanation for the phenomenon: neurons get tired. After firing several times in a row, it gets harder for them to reach activation potential. This is true from the primary sensory neurons all the way back into your brain.
Vision is a perceptual system which experiences a great many different layers of processing between the first physical stimulus and the eventual conscious impression, and only the first and last layers are usually susceptible to conscious control or awareness. We can choose to be aware of the primary information--the color and brightness of individual spots--but we can't help but be aware of much more than that, and that further awareness comes from layers and layers of brain cells that we take completely for granted unless for some reason they're not doing their usual job. Why is it that the world you see seems so three-dimensional, when all each eye really sees is a completely two-dimensional pane of light and color? Something in your brain is thinking hard to extract that information before you're even aware of it. Why is it impossible for you to look at this page without seeing the words? There was once a time when all you could see was the shapes of the letters. How is it that you instantly recognize people you know, even if you sometimes find yourself at a loss to describe them to someone else? Some cluster of nerve cells somewhere is doing that work for you, and you couldn't do it consciously if you tried. In fact, Oliver Sachs describes a Man Who Mistook His Wife for a Hat because that part of his brain was damaged.
Up until now I haven't said anything at all about this particular illusion. That's pretty much covered by the short version.
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