The central theme of the movie is "Carpe Diem" - seize the day. It's what Mr. Keating tells his English students when he takes them to look at photos of long dead alumni from the school. He wants them to make their lives extraordinary, to do some different or bold, to follow their dreams. 

The inspiration for Mr. Keating and for his students is poetry and writing - like Walt Whitman and Henry David Thoreau. When he was a student at the school, Keating and his friends formed a secret group called "The Dead Poets Society" and read poetry together, which is what the boys in the movie do. 

Neil follows his passion for theater. Knox goes for the girl he loves. And the shy Todd will eventually learn to be bold enough to speak up.

But Mr. Keating is considered dangerous for telling them to break free and not to conform, especially in the time period and the school they are in. Being part of the mainstream, being agreeable and just like everybody else is more valued than being unique individuals at Welton. 

When Neil kills himself because his parents will not let him be an actor and are dictating everything about his life, including sending him to military school and then medical school, they look for someone to blame. Mr. Keating and his advice to follow your passions and seize the day are singled out and he is fired. 

The boys - who revered him - are forced to testify against him. They are torn about this decision to turn against someone they loved, and about following like sheep the orders of the school. 

It's the shy Todd who finally dares to speak out - as Mr. Keating is leaving - telling him that they don't actually think he's at fault. The headmaster yells at Todd, tells him to sit down and be quiet - pretty much the same lesson they're taught every day except in Mr. Keating's class. But Todd - bringing together everything in the movie - stands atop his desk, quoting Walt Whitman and saying farewell - O Captain! My Captain! 

Then the other boys who were touched and changed by Mr. Keating's lessons also find the courage to stand up and say goodbye, no matter what the headmaster thinks or the possible punishments. 

So the movie is about seizing the day - making the most of your time before you die - and about being unique and following your passions and being unafraid to stand up for those things.