Generally, the more pixels packed onto a sensor, the higher the resolution of the images it can produce. (Image resolution is somewhat confusingly also measured in pixels, but the term pixels doesn’t always refer to the exact same thing.) A megapixel is 1 million pixels, so a 12-megapixel photo has a resolution of about 12 million pixels. Sounds like a lot, till you consider gigapixel photos, which have over a billion pixels in them.
By comparison, a 30-inch monitor with a 2560×1600 display resolution amounts to a measly four megapixels, and even the best high-definition video currently is around two megapixels, no matter how large the TV.