This is an old typographic unit, known as an 'em square' or 'mutton'. An em square was a square blank type, where the length of each side was the same as the width of an uppercase 'M' – normally the widest glyph in a given font.
It is often used in typography to specify the width of blanks and of different types of dashes. You'll often see references like 'an M/3 space', which means a blank space that is one-third of an em wide.
In CSS, one
em is defined as the current font size, however that is defined. For most elements it's the inherited (computed) value from its parent element. The root element is special, since it doesn't inherit from anything.
1em for the root element is the default font size specified in the browser preferences (16px in most contemporary Windows browsers).
With CSS like the example above, the text size in a paragraph will be 0.75×16px, or 12px.