A Megapixel is the equivalent of one million pixels, and is normally used to reference the resolution capacity in digital cameras.
A singular pixel is a minuscule square, often the size of a "period" on any computerized display, such as LCD or OLED. An average display will hold a grid of pixels with resolutions ranging from 800x600 pixels to 1980x1080 pixels. The more pixels, the clearer the image.
A Pixel, also known as PX or Picture Element, is the tiniest piece of information available in a digital image. Pixels are normally represented by dots or squares and are samples of an original image and will usually need a full screen of pixels to represent the whole image.
Pixels can relate to a large number of contexts, included printed pixels on a page, pixels on a display device, pixels in a digital camera photosensor or even pixels carried by electronic signals. The most notable is using pixels as a measure of Resolution, notably on TVs and monitors.
In regards to digital cameras, where the term megapixel is most common, the picture quality capabilities are measured in the term.
For example, a standard 9 megapixel camera will take pictures with 3672 x 2448 resolution, or 8,989,056 pixels. The image taken will then be an image made up of over 8.9 million dots.
The resolution you want is of course a matter of choice, but it is safe to say that if you want images printed, larger MP cameras are the way to go.