The macula is the center part of the retina responsible for clear central vision. The macula actually occupies only a small part of the overall retina but is critically important to vision. It is the macula that is capable of reading, recognizing faces, driving, etc. The macula is also the part of the retina that allows one to see color.
Diseases that damage the entire retina but spare the macula may be associated with 20/20 vision (although it will appear as tunnel vision). Such diseases include retinitis pigmentosa and glaucoma. On the other hand, diseases or injuries that affect the macula but spare the rest of the retina are associated with terrible central vision, often 20/200 or worse, even though the peripheral vision is unaffected.