Hyperopia

Also known as longsightedness
Genearlly a hyperopic, or longsighted eye is smaller than normal.  Because it is smaller than normal when the eye is fully relaxed light from objects you are looking at is not focused on the retina, but is focused behind the retina, giving blured vision.
Young people can cope with a small amount of hyperopia by increasing the amount of focusing their eye is doing.  However, in older people or those with a large amount of hyperopia the effort to focus is too great leading to blured vision, eyestrain, headaches and poor concentration at visual tasks.
When hyperopia is not causing symptoms it does not have to be corrected, but if it is troublesome then glasses or contact lenses can help with focusing and reducing eye strain symptoms.