Your eyes are amazing things. Like seriously amazing. What happens when you look at an object is that light is focused by a solid, but transparent, mass of protein (called lysozyme) onto the back of your eye. The way in which it is focussed depends on how close the object is. Nearer objects require light to be bent less than distant objects so the lens must adjust it’s shape, a process called accommodation.
For close objects the lens must be short and fat and so a ring of muscle called the cilliary body around the lens relaxes. To make it stretch out again the muscles contract. Problems in your vision usually occur when this lens starts to become opaque, cataracts, or things like myopia (a fancy term for short sightedness) when the proteins of the lens don’t fit together quite right bending light slightly wrong.
I actually have a condition called keraticonnus meaning the front of my eye (the cornea) is cone shaped instead of round. The cornea aso contributes to this focusing effect. Because mine is cone shaped light focus differently depending on where it hits my eye leading to blurred and double vision.