The fovea is the central portion of retina on the back of the eye. The retina consists of light sensing cells, but they are not uniformly spaced, in fact the majority are in a small area towards the centre of vision, the fovea. In practice this means that a very small region of your vision, roughly the size of your thumb at arms length, is seen at high resolution with increasingly less resolution as you move away from it. Your impression of a complete detailed view is constructed by your brain in part as your eye rapidly jumps across the scene (known as saccades) and in part based on memory and past experience.
Used on Chap. 22: page 540