If you're a fiction writer and you haven't checked out Debra Dixon's book Goal, Motivation and Conflict, go do it. Now. This book completely changed my MS, making it stronger, clearer and believable.
Isn't that what we all want in our work?
So what is GMC? Well, the simplest way to explain is to show you first.
Your character's goal is what they are working toward. What do each of your characters want emotionally and physically (internally and externally)?
Next, is motivation. Why do they want to achieve their goal? You should be able to answer this question with "Because...". What drives them toward their goal internally and externally?
And finally, conflict. Why can't they achieve their goals? What is holding them back? Emotionally, your own character is holding themselves back in some way. But why? Physically is simple. For your protagonist, what is your antagonist doing to thwart their goals?
It seems simple, but in my experience, I had more trouble deciphering my character's GMC than plotting my novel!
One last element of my GMC chart is the Dominant Tag (highlighted in pink). Debra Dixon suggests using this to keep your GMC simple. Using an adjective + an adverb will keep your character in focus. For example, my main character is Adelaide. Her brother, Taigen, only wants the best for her, no matter what he has to do. He constantly worries over her and therefore, the tag "Overprotective brother" fits him nicely.
I really recommend finding a copy of Goal, Motivation and Conflict. Not only will it help you get a better reign on your characters, it stresses the importance of each element in order to make your story believable and enjoyable to your reader.