Can some on explain why i need a private in classes?

i have no idea why i need private in classes can someone please explain.When would i use the private: in coding please explain i want to move on but i cant with ought this!
well in professional programming sometimes you dont want to let user access to you literals and data type.because if they can then maby they can damage your raunning programm.
can you give me an example, any code i can look at?
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