Every digital image is made up of pixels. Pixels are little squares of color arranged in a grid. Usually they are so small, you can't tell they are there. Below is an image with very few, very large pixels so you can see them easily.

Now here is an image with more pixels, but there still aren't enough to make them invisible. Can you see them?

Finally, here is an image with lots of tiny pixels. They are so small and there are so many of them you can't see them at all. This image is 500 pixels wide and 334 pixels tall.