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!!!