A megapixel is a collection of 1 million pixels, which are tiny pieces of information that make up a picture. The more megapixels an image contains, the sharper it is.
A common misconception is that more megapixels equal better images. While this is true up to a certain point, it only continues to be true as the size of the image becomes larger. For 8-inch by 10-inch images, after 7.2 megapixels, not much is gained. However, for print, where motion needs to be stopped in low light or incredible detail is required, medium-format cameras are used. As of 2014, medium-format cameras can capture images in excess of 50 megapixels.