What's the meaning of "炉火纯青"?

It means you totally mastered the skills of sth, advanced level .

it means you can make the skill in a very good way.

lú huǒ chún qīng
相传道家炼丹，到炉子里的火发出纯青色的火焰的时候，就算成功了。比喻学问、技术或办事达到了纯熟完美的地步。

You can understand that somebody's learning or some skill is perfection.
例如：她的绘画技术，已经炉火纯青了。
他的厨艺已经到了炉火纯青的地步。

您好!
炉火纯青是一个成语，读音是lú huǒ chún qīng，意思是比喻功夫达到了纯熟完美的境界。原指道士炼丹成功时的火侯。后用以比喻技艺或学问、修养达到精粹完美的境界或做事情达到纯熟的地步。近义词：挥洒自如、滚瓜烂熟、出神入化 。反义词：半途而废、滥竽充数

It is said that Taoist alchemy is successful until the fire emits a pure green color. Later, it is compared to the state of proficiency and perfection in learning skills and self-cultivation。It is high degree of professional proficiency

Tip: just highlight the Chinese characters in cchatty, you will get the English translation.

炉火纯青 lú huǒ chún qīng
lit. the stove fire has turned bright green (an allusion to Daoist alchemy) (idiom)
fig. (of an art, a technique etc) brought to the point of perfection

比喻某一项技能、能力有很高的水平，到达了某一种程度。