Bitmap graphics consist of an 'array of picture elements' known as pixels. This is a bit like a grid with small squares, where each square contains colour. In the following graphic a section of the picture has been magnified so that the pixels are visible. As you can see most of pixels that make up the wall have different colours, whereas the sky is the same colour.
To be more accurate bitmaps should actually be called pixmaps:
"Bitmapped graphics does not use bitmaps (except for purely monochrome images), it uses pixel maps, or pixmaps for short, but the term 'pixmapped graphics' has never acquired widespread currency" (Chapman & Chapman 2004)
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