On class functions, it is a promise that this function will not change any member variables of the object.
There is an exception, and here is the next keyword for you; mutable. When a class member is declared mutable, it can be changed by a function with const on.
The following example code shows these, and has one function that will not compile as it tries to break the const
mutable int y;
void change_x_again() const;
void change_y() const;
// This is fine
// This is NOT fine - it says it is const, but it tries to change the class object.
// This should not compile
void example::change_x_again() const
// This isfine - it says it is const, but the variable it changes is mutable, so can be changed
void example::change_y() const