To be fire: Referring to something in a positive way; a really good, awesome or amazing thing
#1: Two roommates are talking about a friend
Tina: Hey, Cal. Did you see Cheryl at the party today? She has lost so much weight.
Cal: Yes, I talked to her for quite a while. She has also been working out almost every day because she wants to get in shape for her wedding.
Tina: Well, she doesn't need to worry. She looks gorgeous!
Cal: I know! She is fire!
#2: Two friends are eating at a new restaurant
Jason: This restaurant is great! Thanks for inviting me.
Jen: It is. I heard about it from a friend who eats here all the time.
Jason: I don't blame her. This food is fire!
Jen: It's so delicious. We should come here again tomorrow!
Explanation: "To be fire" means that something or someone is really awesome or amazing. Something is really cool if it is fire. In conversation #1, two friends are talking about how beautiful Cheryl looks. In conversation #2, the food at the restaurant is really delicious.