When you send an email to a subscriber and they click 'Reply', the reply message is typically sent to the email address listed in the From: header.
A Reply-To address is identified by inserting the Reply-To header in your email. It is the email address that the reply message is sent when you want the reply to go to an email address that is different than the From: address.
In the example below, 'firstname.lastname@example.org' is the Reply-To address. When a subscriber clicks 'Reply', the reply message is sent to 'email@example.com' instead of 'firstname.lastname@example.org'.