The super obvious answer is that the whole novel is about the Karamazov brothers. It follows the course of their lives from birth to young adulthood, at which point they are all drawn back to their hometown, to the home of their horrible father. The ensuing conflict determines the course of the rest of their lives.
But this is Dostoevsky, and in Dostoevsky's world, things are complicated. Who are the Karamazov brothers? Dmitri, Ivan, and Alyosha share the same last name and are recognized as legitimate sons by their father and by society. Each of them seems to represent some essential aspect of human nature – Dmitri in his sensuality, Ivan in his intellectualism, Alyosha through his spirituality. The title invites us to consider them together; without each other, each brother is only a fragment of a self. The title suggests that the theme of a more universal brotherhood – where we must act toward everyone as if they were our closest kin – is being played out in the story.
But wait – there's more. What about Smerdyakov? No one knows his parentage, but everyone assumes he is the illegitimate son of Fyodor Karamazov. The novel never lets us know for sure. He both is and is not a Karamazov brother. Smerdyakov is the "x" factor in the novel, an unknown quantity but also indubitably, inexplicably evil. Where does he fit into the novel's view of universal brotherhood? Does universal brotherhood also mean accepting, even loving, despicable people like Smerdyakov, as the elder Zosima might argue? The novel certainly makes it tough to find Smerdyakov lovable in any way.
Like much in Dostoevsky's novel, what starts out as a mere matter of fact takes on a more universal, even metaphysical dimension. Other characters such as Rakitin and Kirillovich frequently speak of the "earthy Karamazov force" as if it were the Force, a force that works not just within the novel, but within Russian society and even world history at large.