The main idea of a piece of writing is the one most important thing the writer wants the reader to know.
Remember the one about the turtle and the rabbit? The rabbit, who is really fast, has a race with the turtle, who is really slow. The rabbit breaks out to an early lead. He gets so far ahead that he feels like he can take a break. The turtle just keeps pluggin’ away, step by tedious step, and eventually he catches up. So, the rabbit sprints ahead again, this time getting so far out in front that he has time to catch the new Star Wars movie at the multiplex. Meanwhile, the turtle, who is not a Star Wars fan, just keeps to his consistent though glacial pace, lumbering along, putting one big turtle paw in front of the other. And so it goes. The story ends, of course, when the rabbit, who shot out of the movie theatre like a lightning bolt when he saw that the turtle had caught up once again, and quickly found himself miles ahead of his competitor, decided he was hungry and stopped in at Frank’s Finish Line Diner for a huge plate of chicken fried steak with biscuits and gravy. Now, the rabbit and Frank have been buddies since high school and Frank knows that after a big meal like that his furry little friend likes to stretch out for a cat nap (or is it a rabbit nap?) on the couch in the back room. Well, you can imagine what happened: there’s the rabbit, his big bunny belly full to burstin’ with Frank’s savory vittles, sawin’logs on the couch while that pokey old turtle ambles over the finish line and wins. So what’s the one most important thing the writer of this story wants you to know? Don’t get in a race with a turtle? Don’t see the new Star Wars movie? Don’t order the chicken friend steak at Frank’s diner? Most people say it’s something like, “Slow and steady wins the race.” In this case, the main idea isn’t actually written in the story. But you can figure it out from the key details, the significant things that are in the story that help you understand the writers’message or, as it is sometimes called, the lesson or the moral.