What Is the Reply-To Header?
Reply-To is an email header that tells the recipient's mail client where a reply should be addressed, independently of the From address that identifies the sender.
It solves the central awkwardness of contact form email. You want the notification to be authenticated as coming from your own infrastructure, and you want hitting reply to reach the visitor. Those are different addresses, and Reply-To is how one message carries both.
Forging the From address instead is the mistake this header exists to prevent. It fails SPF, it trains spam filters against your domain, and it eventually lands your own notifications in junk.
One caveat: not every client honours it in every view. Most do, but a few older or unusual clients ignore Reply-To on a reply-all, so do not build a workflow that breaks if the header is skipped.
In WebForms
WebForms notifications are sent From an authenticated address with the submitter in Reply-To, so replies reach the person and authentication still passes.