Eye focusing (technically called accommodation) allows us to see clearly at different distances. Our eyes are designed to focus only at one distance at a time. Normally when looking close, far images will appear blurry. When looking far, near images will appear blurry.
How does focusing work? Our eyes are naturally designed to be clear for distance, so when we look at a near object, the lens in our eye changes shape to redirect light rays on the retina (much like a macro lens on an SLR camera). For near tasks, more effort is required to maintain a clear, sharp image for a sustained period of time.