The first thread which is created inside a process is called Main thread. It starts first and ends at last.
Let's see an example of Main thread in C#.
using System;
using System.Threading;
public class ThreadExample
{
    public static void Main(string[] args)
    {
        Thread t = Thread.CurrentThread;
        t.Name = "MainThread";
        Console.WriteLine(t.Name);
    }
}