If you want to disable an element with pure HTML then you would add a property called disabled :
<input type="submit" name="submit" id="submit_btn" disabled="disabled" />
but if you want to do this dynamically you can use Javascript.
An example where this is needed : you perform an AJAX request and you don't want the user to be able to do several clicks on the button that triggers the request.
A simple implementation would be .. disable the button before triggering the AJAX request and enable the button after your call successfully ended.
Here is the jQuery code for enabling / disabling the button from the code above :
// Disable button $("#submit_btn").attr("disabled", true); // Enable button $("#submit_btn").removeAttr("disabled");