No matter who you are, where you're from, or whether you've interacted with anyone over the age of 60, you've heard that word. It's like "Stellllla!" or "Goooose!" or "Adrian!"
At the end of this movie, we finally, finally find out what all the fuss is about.
Not gonna lie: it's a little anticlimactic. It's not the name of a woman he murdered. It's not the name of a child he gave away after a bad relationship. It's not even the name of his first crush.
It turns out that Rosebud was the name of the sled he used to play with as a child—the same sled he used to shield himself from Mr. Thatcher when the bank man first came to take him away from his parents.
The basic symbolism?
Rosebud reveals Kane's final realization that he's lost his childhood goodness/innocence and become a bad person, since the sled represents a wish to go back to an earlier time in his life before money and fortune corrupted him.
The tragedy is that he only realizes this after it's too late, and his sled Rosebud ends up getting tossed into an incinerator and burned. So it looks like no one will ever realize what Kane meant when he said the name of his favorite boyhood toy.