I have found, through my own experience writing one story for 7 years, that if you can't explain it in one sentence, you're still lost. And if you're lost, imagine what the reader must feel like... :0
No one wants to listen to you talk like a little kid explaining their dream they had last night, right?
"Well, this story is about three little pigs who each build their own house out of something different, and well, the youngest pig builds his house out of straw but then there's this big bad wolf who comes to blow his house down, well, he wants to eat the pigs. So the straw house is blown down and then the youngest pig goes to the second pig's house and that house is made out of sticks and...yadayada blah blah blah."
You probably didn't even read all of that, right? :P
In the beginning it might be like this because YOU still don't know what your story is about.
Is it about the three little pigs' different houses getting blown down? Or is it about the three little pigs getting rid of the big bad wolf? Or is it about some moral here, like...don't underestimate your enemy? Or, if you work together, you can get the job done? Or is that Snow White?
As you're writing and editing your story, you'll probably begin to understand more and more about your story so you can explain in shorter sentences.
It's good practice to sum up your story in less than 300 words. That's the length of a pitch, the little bit a literary agent or publisher will read to see if your story is worth reading or not. It should be short, neat, simple, and to the point. You can't explain last night's dream. A literary agent will read the first sentence of last night's dream and throw your manuscript out the window.
I'm kidding :P But they won't look at it.
Through my own experiences, I have learned that when you are able to sum up your 80,000 word story in 300 words, you know exactly what your story is about. When you know that, you can focus on the most important points and get rid of large chunks of backstory or long scenes with secondary characters.
If it's not about the story, if it's not about your MC, shorten it or get rid of it.
Keep asking yourself;
What is my story about? *finishes draft*
What the heck is my story about? *edits for the third time*
What da heck is my story about? *edits for the sixth time*
What da heck is this ish about, geesh! *edits for the eighth time*
and so on. When you start with "Er, uhm, well it's about a, um..." you still don't know what it is about.
Ask yourself from time to time like a pop quiz.
"Hey, me, what is this story about?"
If you answer right away and the answer is short and simple, good! You know your ish.
If you can't and you stumble and it takes a long time to get to the point, BAD! You don't know your ish. You still need to figure it out, friend.
But don't worry. Don't fret. Take a deep breath and have a cup of coffee. Look at your story with fresh eyes and remember that first day you started to write your story. What were you thinking when you started? What did you want to write about? Think about those things and about the overall theme of your story. That should get you started :)
Now, let me emphasize one more time: Ask yourself while you're writing and editing, "What is my story about?" Don't forget the core part of your story. Don't stray too much. But also, don't worry, you've got this! :D