The lens in each eye gathers light by bending it so the image focuses on the retina. Our lenses can deform so that objects at different distances come into focus.
When your hand is too close to your face, the lens in your eye can’t deform enough to see it clearly. We need to adjust the focus by placing a glass lens in between our eye and the object.
Glass lenses bend the light in the same way as the lens in our eye does but they can only focus at one distance at a time.