Visual Basic is event driven programming language. Over here cmdPush_Click() is a event procedure, which belongs to cmdPush command button object.
Visual Basic automatically names your event procedures. The name consist of the object name, an underscore(_), and the name of event. For example: the click event of command button called cmdPush will be cmdPush_Click().
This procedure will be executed when ever Click event of the cmdPush command button is fired.
When you will click on the cmdPush command button at the run time the statements with in the cmdPush_Click() event procedure will be executed.