Explanation 1 (Very simple):
Light goes into your eye through a tiny hole called the pupil. The lens helps focus the light. At the back of the eye the retina makes a picture, but the picture is upside down. Nerves send that picture to your brain, and the brain turns it the right way up so you can see.
Explanation 2 (Like a camera):
Think of your eye like a camera. The outside scene is the light. The light goes through the pupil and lens, and the retina is like the camera's film that catches the picture — it ends up upside down. The nerves are like wires that send the picture to your brain, and the brain flips the picture so you see everything the right way around.