Literally on fire means in the process of burning:
The factory was on fire.
But the expression on fire is also used in figurative sense. It may mean:
1) feeling very strong enthusiasm, love, passion, etc.
2) very successful
Here are some examples with the two figurative meanings:
I didn’t want to tell him that what he was doing was in vain because he was on fire with enthusiasm.
I like the way they look at each other, those two lovers with their hearts on fire.
Jake is on fire today, he already scored seven points.
You did really well in the game. The team was on fire today.
He really is on fire, I don’t think anything or anybody could stop him now.
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