Type is usually measured in picas and points ... but horizontal spacing is measured in ems, and the em is a sliding measure. One em is a distance equal to the type size. In 6 point type, an em is 6 points; in 12 point it is 12 points ... Thus a one em space is proportionally the same in any size.

The book then shows a little diagram with several different sized squares.

So, as I understand it, one em is the width equal to the height of a font (which is generally, but not always, the width that the letter m is designed as). What your quote is saying, then, is all the letters fit in a box which is twice as tall as it is wide.