The 'Use Reply To' option is use to either have the recipient of your email to reply to the actual sender of the email or reply to a specify address of your choice.
In the 'Mail Preferences' page, select the 'Use Reply to' field to modify the Reply to settings. The options you have for modifying the reply to settings are
'Do not specify a reply email address': When you select this option, no reply-to is used in the mails you send.
'Mail Recipients should reply to the address below': When you select this option, the reply-to in the sent message will be populated with the email address written in the Reply-to address textbox below this field. When you select this option, you have to enter an email id in the Reply-to address text field.