This means that the latin character width is roughly half of character height, so "full-width" square CJK ideographs fits exactly as the width of two "half-width" latin characters.

(Wider monospace fonts would typically require extra character spacing between CJK ideographs.)

Ref: http://www.w3.org/TR/clreq/#basic_features_of_chinese_script ("squares with the ratio of 1:1, and are seamlessly arranged with one another.")