Well, 'burn baby burn' gives the impression of something exciting or chaotic. In the context of a TF2 fanfic, it could be about a situation where the characters are in a really tough fight. Maybe there's a lot of destruction and high - energy action. For example, it could be about a battle between the two teams where the environment is on fire and the characters are fighting tooth and nail to win. It could also be about a character's personal journey within the game's world that has this 'burning' or intense aspect to it, like a character facing a really difficult challenge that they have to overcome with a lot of passion and determination.
I'm not sure if it's popular. There are so many TF2 fanfics out there, and without more information or some kind of popularity metric like views or reviews on a particular fanfic site, it's hard to say.