What’s an em space?
In handset type, a metal space that is the square of the type body size; that is, a 10 point em space is 10 points wide. The em gets its name from the fact that in early fonts the M was usually cast on a square body.
A Basic Course in Typography by James Craig
An em is a unit of measurement in the field of typography, equal to the point size of the current font. This unit is not defined in terms of any specific typeface, and thus is the same for all fonts at a given point size. So, one em in a 16 point typeface is 16 points.
Wikipedia
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... and in 60 point type an em is 60 points. Thus one em space is proportionately the same in any size.
The Elements of Typographic Style by Robert Bringhurst
The em unit is equal to the computed value of the font-size property of the element on which it is used. The exception is when em occurs in the value of the font-size property itself, in which case it refers to the font size of the parent element. It may be used for vertical or horizontal measurement.
World Wide Web Consortium (W3C)
