Tracing a picture in Photoshop is a way used to create a vector graphic from a raster picture. A vector graphic is made up of mathematical curves and features, which makes it infinitely scalable with out dropping high quality. A raster picture, alternatively, is made up of pixels, which may grow to be distorted or blurry when enlarged.
Tracing a picture might be helpful for a wide range of functions, akin to creating logos, illustrations, or web site graphics. It may also be used to transform hand-drawn sketches into digital information.