A bitmap font is a font with jagged edges when enlarged, instead of a scalable font where no matter what the size will look just the same. Bitmap fonts are comprised of several dots that make up a single character.
The picture above is an example of scalable font vs. a bitmap font. As you zoom in on Computer Hope, there is no degradation in quality with a scalable font. However, it's the complete opposite when we zoom into a bitmap font.