First of all, P=NP is not a "theory"; it's a _question_. We do not know whether the answer is "yes" or "no"; most experts suspect that the answer is "no", i.e. P is not NP. There were attempts to prove it both ways, none successful or even remotely close. The theory encompassing the question is called computational complexity. It is a relatively new field (foundation is often taken to be ~1965), and many questions are open. However, we do make progress and each year there are breakthroughs. People hope to solve it some day - history shows that many open problems in mathematics resisted for long, but they were finally solved after centuries of thinking.
"I want it in simple words so it is easy to understand"
The definition of P and NP uses things such as "decision problem", "polynomial running time", "nondeterministic" algorithm or Turing machine etc. You need effort to understand them. You have to read about those prerequisites, internalize them in your mind, and only then it will become understandable. "Simple words" won't help in overcoming that. You might have heard lots of intuitive musings about P=NP and are not satisfied. The only cure is rigor.
I hope you will not take my words as condescending, but it is not helpful when you say "it makes no sense to me"; it's like someone told you "the program does not work", how are you going to help that person? You need to have a leg to stand on. What you do understand from explanations, what do you not understand from them?