In a novel, motivation is the engine that drives your plot. WHY does Sabrina act the way she does? WHAT causes her arch-nemesis, the Zombie King, to relentlessly pursue her?
It is your job as author to know and communicate the answers to those questions. Another way of looking at your character’s motivation is to ask: what does she want? The answer to this question is one of the six elements of a plot, and it must be answered. Whatever your character most wants must be important, and failure to achieve it must have serious consequences.
Without something important at stake, your main character’s actions may seem random and readers won’t identify with her. You must also be aware of the motivation of the main secondary characters.
Several years ago, when I was writing a “horror” novel for my young adult series, Phantom Valley, my editor and I got into a lengthy argument because we disagreed on why a ghost was haunting the main character.
That’s how important motivation is: we couldn’t agree on the resolution of the plot until we resolved the motivation of the ghost!
Note that motivation is not the same thing as conflict. Motivation is the WHY; conflict is often the WHY NOT. A good example can be seen in my first science fiction novel, Pandora’s Genes. At the beginning of the story we know that Zach’s primary motivation is to carry out the order of his leader, the Principal, to purchase and deliver to him a young girl, Evvy. But there are also important conflicts that bear on this motivation. For one thing, the Principal’s directive goes against Zach’s own moral code. Other conflicts include those imposed by the environment (poison bats) and other characters (the brigands who want to kill Zach and kidnap Evvy).
Zach’s moral trepidations here are a good example of inner conflict; the poison bats and brigands are examples of outer conflict. A complex, well-thought-out character will be faced with many examples of both sorts of conflict.
A rule of thumb is that resolution of inner conflict results in a change in attitude, while solving an outer conflict results in a change in circumstance. In Pandora’s Genes, Zach comes to see that he was more complicit than he had believed in the Principal’s wrongdoing; while he eventually overcomes all the outer conflicts and returns home to a situation that is changed by his having resolved the inner conflict.