Overriding vs method hiding

I am a bit confused about overriding vs. hiding a method in C#. Practical uses of each would also be appreciated, as well as an explanation for when one would use each.

I am confused about overriding - why do we override? What I have learnt so far is that by overring we can provide desired implementation to a method of a derived class, without changing the signature.

If I don't override the method of the superclass and I make changes to the method in the sub class, will that make changes to the super class method ?

I am also confused about the following - what does this demonstrate?

class A
{
    virtual m1()
    {
        console.writeline("Bye to all");
    }
}

class B : A
{
    override m1()
    {
        console.writeLine("Hi to all");
    }
}

class C
{
    A a = new A();
    B b = new B();
    a = b; (what is this)
    a.m1(); // what this will print and why?

    b = a; // what happens here?
}