RequiredField validator in asp.net

What is RequiredField validator in asp.net?

  You can use it to make sure that the user has entered something in a TextBox control. Let's give it a try, and add a RequiredFieldValidator to our page. We will also add a TextBox to validate, as well as a button to submit the form with.

<form id="form1" runat="server"> 
 Your name:<br /> 
<asp:TextBox runat="server" id="txtName" /> <asp:RequiredFieldValidator runat="server" id="reqName" controltovalidate="txtName" 
errormessage="Required" /> 
<br /><br /> 
<asp:Button runat="server" id="btnSubmitForm" text="Button" />

 </form>