Well let's start with the Kane part of Citizen Kane, since it's the part of the title that's taken from our main character's name. You might already know that Kane is a famous name (also spelled Cain) that comes from the Book of Genesis.
Cain is the name of a dude who kills his brother because of jealousy and pride. He also ends up getting banished into the wilderness forever. And you can see a parallel in Charles Foster Kane, who throws away his best friend Jedediah (also a Bible name) in order to pursue his insane thirst for power. By the end of the movie, Kane is completely doomed to isolation, and it's only in his final moments that he realizes he isn't a good person at all (like Abel), but a bad dude like Cain.
Now then, the "citizen" part of Citizen Kane has a less obvious significance. But the meaning starts to emerge if you think about what it means to be a citizen. Being a citizen means that you're part of a community, and that you have certain responsibilities toward that community.
This is exactly what Kane think of himself early in the film, as he tries to use his power and fortune to be responsible to his community and to attack the corruption of rich people and politicians. But over time, Kane forgets that he is a "citizen" in the moral sense and spends all his time only thinking about himself. Ultimately, Charles Kane actually fails to be the good citizen he always wished to be.