Many
people (especially ages 9-16) use the expression "That's Fire!" a lot. What they usually
mean by that is
something is so awesome that it is like fire. Or, rather, something is so cool that it is on fire (figuratively). So, to sum it all up, the phrase "That's Fire," is used for awesome things.
Ben: I made a story! Guess what the main character's name is?
Larry: What?
Ben: Bob!
Larry: Wow! That's FIRE!!!, bro!!!
Ben: Yeah. Guess what he does in the book?
Larry: What?
Ben: He eats DINO NUGGETS!!!
Larry: Bro, that's SUS!!!