Light travels into your eye through the dark hole called the pupil. The lens inside your eye bends the light and makes a tiny picture on the back of your eye (the retina). That picture is upside down. Little nerve messengers carry the picture to your brain, and your brain turns it the right way up so you can see.
Think of your eye like a little camera. Light comes in, the lens helps focus it, and a picture is made on the back of the eye. The picture is flipped over there, so the brain acts like a clever computer and flips the picture the right way so you know what you are looking at.
Imagine tiny light bugs rushing into your eye through the pupil. The lens is like a magnifying glass that helps the bugs paint a picture on the back wall (the retina). The picture looks upside down, so the nerves send that upside-down picture to your brain. The brain turns the picture the right way up, and then you see things correctly.