The PlaceHolder server control works just as its name implies — it is a placeholder by which you insert objects automatically into your page. It acts as a marker with which you can add other controls.
Example of PlaceHolder Server Control.
Insert a PlaceHolder control into your page and then add controls to it from your server-side code. This example creates a new instance of a Label control and populates it with a value before it is added to the PlaceHolder control. You can add more than one control to a single instance of a PlaceHolder control.