const_cast: modifying a formerly const value is only undefined if the original variable is const

https://stackoverflow.com/a/332086/462608

modifying a formerly const value is only undefined if the original variable is const

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if you use it to take the const off a reference to something that wasn't declared with const, it is safe.

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This can be useful when overloading member functions based on const, for instance. It can also be used to add const to an object, such as to call a member function overload.

I am unable to understand the meanings of the above quotes. I request you to give me examples to practically show what these quotes mean.