Light goes through the pupil, the dark hole in the center of your eye. The light becomes an image on the retina, which converts these images into nerve impulses. The brain reads these impulses and "tells" you what you are seeing. The image that the retina receives is upside down, but the brain reverses it to right-side up.
In an experiment years ago, a brave researcher wore special lenses that turned everything upside down. After stumbling around for awhile, he became used to the lenses because his brain corrected the images. To conclude his experiment, he stopped wearing the lenses. After awhile, his brain returned his vision to normal.
People with vision problems are usually near-sighted or far-sighted. Near-sighted people see clearly only objects that are near; far-sighted people see clearly only objects that are far away. Both groups of people need corrective lenses (glasses or contacts).
People whose near and far vision is good have "20/20" vision. People with poor eyesight might have 20/40 vision (or worse). That is, a person with good vision can see an object from 40 feet away as clearly as a person with poor vision can see the object from 20 feet away. No matter how good your vision is when you are young, you will need reading glasses when you get old.
Your pupils get bigger as a light source gets dimmer, but your pupils also get bigger if you see something (like a delicious meal) that interests you. "Your eyes are bigger than your stomach" is an expression parents use when a child fills his plate with more food than he can eat.