Humans have two eyes that are horizontally offset. In order to look exactly at an object with both eyes and create a stereoscopic image, our eyes must converge or diverge a given amount. If they do not, the brain receives images that don't align, and we see double vision (diplopia). The eyes can converge too much, too little, or not at all. They may converge different amounts at far and near, creating a problem at one or both distances. Only severe convergence issues result in diplopia. Prior to that, you may experience sleepiness when reading, headaches, eye strain, dry eyes, blurred vision, a "pulling" sensation, or general discomfort.
Convergence disorders are variously treated with reading glasses, prism lenses, and vision therapy.