Sight-reading complex musical notation takes years of training, hundreds or thousands of hours of practice, sitting in front of the piano, a metronome drilling its infernal clicks into your brain. Eventually you'll gain the ability to read and perform just about any piece of music that's set in front of you, without ever having seen it before. It does not come easily, and it is not a natural skill; you have to keep practicing to retain it. Each month you don't practice takes two months to earn back the power you've squandered. It is a human achievement, a way in which we force our brains and fingers and feet and eyes to perform a task we are not born able to do.