What it means
It changes the texture and width of a word or line. A little additional spacing can help certain short uppercase labels, while tighter spacing can change the feel of a large heading. The right value depends on the font, case, size and surrounding whitespace.
Use it in practice
Tune the actual text at the size where it will appear. Recheck line breaks when tracking changes the width of a heading.
Watch out for
Spreading lowercase body text heavily can make words harder to recognize. Do not use tracking to compensate for the wrong font or a cramped layout.
