: the condition of being eager to act or work : the condition of being motivated
: a force or influence that causes someone to do something
Motivation, as it pertains to writing, can take two forms. First, there's character motivation. I'm going to focus on villains here, because they usually get written as bad for the sake of being bad... and that's not always the best, most powerful, or most plausible way to write a villain.
Understanding your villain's motivations with regard to their actions will not only help you to decide what they do next, it will help your readers understand WHY they do what they do. Readers find "WHY?" very important, and if they don't understand "WHY?" they're liable to lose interest in your story relatively quickly.
It is possible to write a character that has no discernible motivations for their actions (take Iago in Othello
Instead, show us why your villain is bad. What does he stand to gain from his actions? Give us a snapshot of what happened in her past that made her the way she is. Take Voldemort, for example. Throughout the entire Harry Potter Series
In its other sense, MOTIVATION applies to you, dear writer. Find your story. Make it something you're enthusiastic about. Because in order to succeed with your story, you're going to be spending a LOT of time with it. And if you're not motivated by the sheer joy of being with your story, you're going to find the process of writing, editing, and publishing a very arduous one indeed.
Good luck and happy tales.