What it does

"Improves the quality of a Skill Gem." Each skill has its own quality bonus, and quality is capped at 20% by default.

How players use it

Because the bonus differs per skill, it's worth checking what quality does for your main skill before buying. Patch 0.4.0 reworked several: Ice Shot's quality became more Magnitude of Chill, and Incinerate's became more Ignite duration.