What's the meaning of "it's not the whole story"

It is a phrase that is often used when someone is presenting an argument or is recounting a story. "It's not the whole story" means that the person is omitting an important aspect of the argument or the story, whether inadvertently or deliberately. For example:

ME: There are 30 students in my class and I'm the only one who failed the exam. This is proof that the teacher dislikes me. It's not fair that he should give me a bad grade just because he dislikes me. I should complain to the principal that my teacher is biased against me.

YOU: That's not the whole story. You're neglecting to mention that you were the only one out of the 30 students who didn't bother to study for the exam.

Thank you so much for the explanation.

IT is also used when someone tells you something and you know they are either unaware of or are hiding details. Generally, they are unaware of part of the story.