The 'public' keyword is an access modifier. It makes a method visible to the code that lives outside of the class that holds the method that is marked by the 'public' keyword.
In the example below there are two classes. The class Progam contains a method Main. When this method gets called, it creates a new radio object and stores it to myRadio variable. Then it calls the method TurnOn on the object that is stored in the myRadio variable which will then turn the radio on.
The method TurnOn() is defined in the Radio class and it can be called from within the class Program because it is marked as public by the access modifier 'public'.
Now have a look at the method TurnOff(). This method doesn't have the access modifier 'public' so it won't be possible to call it from the method Main of the class Program.