Projections are a method of showing the surface of a sphere (globe) on a flat surface. Since the surface of the Earth is a uniformly curved surface (of a sphere) and paper is flat it is the only reasonable method of drawing a map.
The Mercator projection shows lines of longitude and latitude to be parallel. Since lines of longitude meet at the poles, the distance between lines (for instance) 15 degrees apart, becomes smaller as they approach the poles. On the Mercator projection, that convergence isn't shown, therefore the further from the equator, the more features appear larger than they really are. Most Mercator projections also expand the distances between parallel lines of latitude as they get further from the equator. That has the effect of both magnifying the effect of features appearing larger than real and causing their shape to appear MORE accurate. That's kind of like using a "zoom" effect.