startActivity() method is called when a new activity is started. Intent object is passed as parameter and it describes your activity. What actions you want to perform are described by this intent. Also data used by an activity is carried by this intent.
For launching a new activity, create an intent that describes the activity using the class name. Following code explains how one activity can start a new activity.
Java Code: This is the code to create a new Intent
Intent intent = new Intent(this, SignInActivity.class);