Drag the guides so they match the font
"Cap Height" is the height of the capital letters. "Baseline" is the bottom of the letters. "X-Height" is the height of the lower case x. "Descender Height" is the height of dangling characters like y, j, p and g. Align the guides (the solid pink line) to get the values.
NOTE! This is an improved version of the cap height calculator found here: https://codepen.io/MisterCurtis/pen/doEzzx