Confirmation on my Understanding of Beginning Android Concept (Using Multiple Activities)

I am a beginner at android development, and have reached the end of Building your First App. Before I move on, I would like to confirm and validate my understanding of using multiple activities and communicating from one activity to another.

/** Called when the user clicks the Send button */
public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText) findViewById(R.id.edit_message);
    String message = editText.getText().toString();
    intent.putExtra(EXTRA_MESSAGE, message);
    startActivity(intent);
}

1) Is my understanding correct that the second paramater in the constructor for Intent (Intent intent = new Intent(this, DisplayMessageActivity.class)) serves as a reference for startActivity(...) and reflection is used to call the method onCreate() in the class DisplayMessageActivity since the class DisplayMessageActivity was given as a class object?

2) What is the use of the first paramater (the context one in the constructor)? (Basically how does Android use the first parameter, a brief description please, to start the activity)?

3) As seen in the tutorial, last part of building your first app, it advises me to declare a variable as such: (public final static String EXTRA_MESSAGE = "me.marshac.myfirstapp.MESSAGE";). I know the purpose of this declaration and initialization, but why don't I have to specify the full package name (me.marshac.myfirstapp. (...) .MESSAGE) and where is the MESSAGE variable coming from? The only two variables similar to that are both local variables in sendMessage() and the other activity's onCreate(), but they are different case and local?

I am sorry for the very in-depth inquiries, I want to establish a firm understanding of the beginner concepts before I move on to intermediate/advanced ones.