what is fovea or yellow spot ?

The yellow spot is a small depression forming a shallow pit in the retina at the back of each eye in the human body. It is slightly yellow in appearance and so was first called the "Yellow Spot" .The term "yellow spot" is another word for macula lutea or fovea: Different terms are used in different texts but the meaning is the same. Because it contains a large number of the light-sensitive photo-detector cells called cones, the yellow spot ("macula lutea", or "fovea") is the area of greatest acuity of vision.
This means that when an eye is directed at an object, the part of the image of that object formed on the retina that falls onto the fovea is the part of the image that will be perceived in the greatest detail.