Serious Business is when a story revolves around an activity where a sizable portion of the In-Universe population takes it far more seriously than they should. If the popularity of some mundane object rivals that of Elvis Presley, The Beatles, and Michael Jackson combined, or if there are mainstream schools devoted to it instead of teaching reading, writing, and arithmetic, then it's Serious Business.
It's often a Silly Reason for War and can turn a casual conversation into a heated "Cavemen vs. Astronauts" Debate. Someone who just can't let it go may become a Single-Issue Wonk. If competition is involved, you can almost guarantee that Second Place Is for Losers. Frequently "opposed" by the Cavalier Competitor. Compare "Stop Having Fun" Guys. In musicals, it's often demonstrated that Dancing Is Serious Business. For the saying that "The Internet Is Serious Business", see G.I.F.T. Related to I'm Not Here to Make Friends. Not to be confused with Mundane Made Awesome, though the two tropes overlap plenty.
Compare Duels Decide Everything, Comical Overreacting. When a relatively minor crime is treated as Serious Business, that's Felony Misdemeanor. Contrast Not a Game and Matter of Life and Death. A Super-Trope to Overdramatic Dating Commotion, where dating or the mistaken thought of it is treated like it's a huge deal.
Serious Business (Trope) has been suggested to play 2 roles. Click below to see other actors suggested for each role, and vote for who you think would play the role best.