One of the attributes that you can set for any form element using Drupal's FormAPI is "#disabled". This effectively disables the form element - in most browsers, the user actually sees the element "greyed out" and is unable to set focus on the element.

When a form element is disabled, there is the sometimes unintended consequence of the data from that field not being sent back to the server when the form is submitted. For forms that were initially blank (like new node forms, for example), this usually isn't a problem. But for "edit" forms, this can be a dealbreaker - this problem sometimes shows it's ugly head when a developer tries to use the "#disabled" attribute to add an additional layer of security to the form - forcing the user to click an "edit" link that users jQuery to un-disabled the form field.

A better solution to utilize when you want this extra layer of "are you sure"-type security is to utilize the "readonly" HTML attribute. Like "disabled", "readonly" makes the field uneditable, but unlike "disabled", the values are passed back to the server when the form is submitted. Unfortunately, there is no Drupal-equivalent, so to implement it, you'll need to do something like this:

$form['my_textfield']= array(
'#type' => 'textfield',
'#title' => t('my textfield'),
'#size' => 20,
'#attributes' => array('readonly' => 'readonly'),
);