The x-height refers to the distance between the baseline and the top of the lower case line in a typeface. Typically, this is the height of the letter x in the font (the source of the term).
The x-height is a relative measure of typeface, so different typefaces set in the same point size may appear differently. A typeface with a large x-height looks much bigger than a typeface with a small x-height at the same size. On the illustraton below, all letters were set in 72 points, yet resulting in different heights of the lower case. The relationship of the x-height to the em square defines the perceived type size.
1 Brioni; 2 Fedra Serif A; 3 Greta Sans; 4 Irma Text; 5 Jigsaw; 6 Greta Text; 7 Fedra Sans; 8 Nara; 9 Charlie
The point size (e.g. 72 pt) refers to the em-square of the typeface. All relative proportions of the typeface are determined within the absolute size of the em-square. The em-square unit is the same in all fonts at a given point size. Proportions of fonts (descender height, x-height, cap height, ascender height) are defined by the characteristics of a typeface and change from font to font.