System.Windows.Forms.Timer Control is essentially a new thread that notifies you on regular intervals. Its notification happens through an event called "Tick".
So when you say mytimer.Tick += new EventHandler(this.mytimer_Tick)
you are subscribing to that event. You say, "when your time (interval) has elapsed, I want to know about it"
When timer ticks - it calls
private void mytimer_Tick(object sender, EventArgs e)
{
//and this is where you do the work that happens every interval (Timer Tick)
}