Some people say that Charles Darwin's theory is not true. Can you explain it to me?

I believe that the question is asking for clarification on Darwin's Theory of Evolution vs. asking for clarification on why people might not believe it. 

Often, people assume that Darwin was the first person to think that species evolved and changed over time.  That is simply not true.  Jean-Baptiste Lamarck proposed the first fully formed scientific theory of evolution in 1809.  That’s 50 years before Darwin’s On the Origin of Species. Darwin’s theory obviously...

I believe that the question is asking for clarification on Darwin's Theory of Evolution vs. asking for clarification on why people might not believe it. 

Often, people assume that Darwin was the first person to think that species evolved and changed over time.  That is simply not true.  Jean-Baptiste Lamarck proposed the first fully formed scientific theory of evolution in 1809.  That’s 50 years before Darwin’s On the Origin of Species.  Darwin’s theory obviously gained more popularity in the long run than Lamark’s because of Darwin’s explanation for how the mechanism of evolution possibly works.  Darwin’s theory involves things like adaptation, fitness, survival of the fittest, and natural selection.  I’ll take each one in turn.  

Through normal genetic changes that result from sexual reproduction and mutations, organisms exhibit small changes from generation to generation.  Some of those changes allow a particular organism to be better adapted to a particular environment.  An organism that has a beneficial adaptation has a higher level of fitness, and that gives the organism a better chance to survive. That’s why it’s called the survival of the fittest.   A surviving organism is more likely to reproduce and pass on its genes.  That means the next generation will likely have the beneficial adaptation.  A small change has now occurred in a population of organisms.  To an observer, it appears as if nature has naturally selected the better adapted, more fit, organisms to survive and pass on their traits.  Darwin said that if given enough time, these small changes will add up, and large changes will occur across a species.  The changes would be large enough that the current species no longer resembles the original, and a new species has been born.  Darwin’s theory explains how species can greatly evolve through small changes.  

As for why people might not believe it, in my experience it deals with their religious convictions.  For example, some Christians believe in a literal interpretation of the Bible.  That means that Earth and the entire universe were created in six, 24 hour periods.  That argument is continued throughout the Biblical history, and some people then believe that Earth is just over 6,000 years old.  They don’t believe in Darwin’s theory, because it requires a lot more time than 6,000 years to have passed.