Blinded by the Light

Being a scholarly man, I have naturally abhorrent eyesight. My visual acuity is about 20/500, in both eyes. When compared to the perfect eyesight of 20/20… I’m not doing too hot. But I wondered, as I put in my contacts for astigmatism, what that number actually measured. What does it mean? How is my eye so horribly out-of-whack?