What it means
It describes the main body height of many lowercase letters, excluding ascenders and descenders. Two fonts set at the same nominal size can look noticeably different in scale because their x-heights differ. This matters when pairing families and when choosing text for small labels or paragraphs.
Use it in practice
Compare lowercase text at the actual size rather than matching only the numeric font-size value. Adjust paired fonts optically when necessary.
Watch out for
A larger x-height does not guarantee better reading in every context. Spacing, shapes, weight and line height still matter.
