When Sending a Mailing the Reply-to email is different than the From Address
This is a mechanism for handling bounce messages and replies in response to the mailing so that they are associated with the individual that mail was sent to. It is quite complicated and you should start by reading https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/ and probably also the Sparkpost documentation as well. Too much to explain here. I presume that you didn't set the Sparkpost up. You could talk to whoever did.