When an auto-reply is used with the standard contact form, the "From" on the auto-reply email is set to the value entered as the recipient. In some cases (e.g. email to the CEO), it wouldn't be good to reveal that email address. Using this module, you can choose to hide the recipient's email in the auto-reply. It will instead come from the default site email address. This can be set separately for each contact form category.
To use Contact Hide Email, just install and activate it as you would any other module. Each "Edit contact category" page will then include an option to "Hide recipient's email address in auto-reply". Checking this box (or choosing "Yes" in the D6 version) will make that auto-reply email come from the site default email instead of the recipient's email.