'On fire', literally, means that something is in flames.

on fire，字面意思是某物着火了。

But in Indonesia, extremely dry forests are in flames.

但是在印度尼西亚，极度干旱的森林正在燃烧。

When we reached Mandalay it was in flames.

【flame】我们到达曼德勒时，那里一片火海。

Why am I writing a memoir about fitness when the world is in flames?

为什么我要在世界大火时写一本关于健身的回忆录？

The skin over his cheekbones was tighter and there was a flame deep in his large smoky eyes.

他颧骨上的皮肤更紧了， 烟熏味的大眼睛深处燃烧着火焰。

Well, one meaning of " to be on fire" is that you are burning or that your clothes are in flames.

好吧，“着火” 的一个意思是你在燃烧或者你的衣服着火了。