How exactly do "Objects communicate with each other by passing messages"?

In several introductory texts on Object-oriented programming, I've come across the above statement.

From wikipedia, "In OOP, each object is capable of receiving messages, processing data, and sending messages to other objects and can be viewed as an independent 'machine' with a distinct role or responsibility."

What exactly does the statement mean in code?

class A
{ 
    methodA()
    {

    }
}


class B
{
    methodB()
    {

    }
}


class C
{
    main()
    {
        A a=new A();
        B b=new B();
        a.methodA(); // does this mean msgs passing??
        b.methodB(); // or does this?? I may be completely off-track here..
    }
}