This means that you can’t post values containing HTML or script tags.  So when you have an ASP.Net page with a textbox on it, and your users enters something like

And then the user press a submit button, they get this error.
More info about this can be found here.

Just change the page-directive on top of your page by adding the tag "validateRequest".  Set this tag to false and your users are able to add HTML tags in your textbox.