This Required Field Validator forces the user to enter text into the control associated with the validator. If you want to make sure the user changes a default value placed into the control, set the control’s Text value and the Initial Value property of the Required Field Validator to the same value.The associated control is set by right clicking on the validator and selecting Properties from the context menu. The rest of the validation controls provided by ASP.For example, you don't want to process a form that's missing critical information.When users enter values into an HTML form, the values that they enter are strings.You can read more about validation with Ajax in this excellent tutorial on j Query For Designers.
Drag one Text Box control and give it an ID of First Name Text Box, two Button controls (Submit Button and Cancel Button), and one Required Field Validator (First Name Required Field Validator). The following screen shows the form in action, and the user has pressed the Submit button without entering any text into the textbox. The Required Field Validator fails when the value in the associated control matches the value of the Initial Value property of the Required Field Validator.
In this control we can validate and compare two input variables based on their data type using the comparison operators like equal to, not equal to, greater than, les than etc.