what is event handler and how to add it?

An event handler is a routine that handles an event.

The easiest event to understand is the button press event, if you insert a button into a dialog, then you will want to react to the event of pressing that button.

The event handler (typically added from the dialog editor or the class wizard) will take care of that event and using it's parameters allow the programmer to filter and process what is needed to be handled in that event.

in pseudocode:

OnBtnClick()
{
  MessageBox("you have pressed the button");
}

Take a look at this wikipedia article[^] which explains it very well (from the event itself to the event handler).

Good luck!

See below articles

Step by Step: Event handling in C#[^]

Events and event handling in C#[^]