Motivation, as defined by Merriam Webster: the act or process of giving someone a reason for doing something : the act or process of motivating someone
: 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, for example). We're not all Shakespeare, though... And even Iago's motivations can be teased out of the play if you want to look VERY deeply into it.
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
his most simple and immediate goal has been to kill Harry. Understanding WHY is very important, though, for Voldemort's actions to make any sense to the reader. Also, Harry didn't initially have to understand WHY Voldemort wanted to kill him. But in order to eventually defeat Voldemort, Harry would have to learn everything he could about his nemesis's motivations.
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.