Selecting the "Improve Delivery" option for a from address will use a Savvy Sender from address instead of your from address to improve delivery.
For example, if your email address is "firstname.lastname@example.org", then the from address used when sending out messages, campaigns, and emails will be "email@example.com".
This helps to prevent emails from being marked as spam, because instead of sending the email on behalf of your email address, the email is actually sent, authenticated, and validated as an email from a Savvy Sender owned domain (send.savvysender.net) using DKIM and SPF.
Replies will still be sent directly to your actual email address, because improve delivery does not affect replies for messages, campaigns, and emails.
Improve delivery is automatically set for certain email domains, such as @yahoo.com, because Yahoo! no longer allows third-parties to send email on behalf of their users.