Your sight is your ability to see. If you're maneuvering through the house at midnight using your shins to locate the furniture, you'll appreciate your sight that much more. You could also turn a light on.
Sight has many different senses. It can mean the range of your vision, as in when you freak out when your little puppy goes out of sight. You can "take in the sights," meaning you're seeing all the special attractions of a town (also known as sightseeing). Sight can also be a verb, as when you sight a celebrity at your local taco stand, or when you sight your target using the sight on your gun, before you shoot.