Jessica is experimenting with bitmap images, but she accidentally zooms in too far and sees many small colored squares. jessica is viewing the image.

Jessica is viewing the pixels of the image.

A pixel is a point that represents the smallest element that constitutes an image. Every pixel has its one color, luminosity, contrast and so on. They can't be seen with a naked eye because they are too small, but if you are zooming in enough on an image, they appeared to you (or to Jessica, in this case) in forms of many small colored squares.