How to send string from one activity to another?

You can use intents, which are messages sent between activities. In a intent you can put all sort of data, String, int, etc.

In your case, in activity2, before going to activity1, you will store a String message this way :

Intent intent = new Intent(activity2.this, activity1.class);
intent.putExtra("message", message);
startActivity(intent);

In activity1, in onCreate(), you can get the String message by retrieving a Bundle (which contains all the messages sent by the calling activity) and call getString() on it :

Bundle bundle = getIntent().getExtras();
String message = bundle.getString("message");

Then you can set the text in the TextView:

TextView txtView = (TextView) findViewById(R.id.your_resource_textview);    
txtView.setText(message);