The 'Referrer Policy' header controls what information is passed on to the next site whenever a link is clicked on your site. It's purpose is to prevent 'Reverse TabNapping' used in phishing attcks.
If an external link has the target="_blank" attribute vaulue it provides partial access to the referring page via the 'window.opener object'. A Referrer Policy helps to prevent phishing attacks by restricting access to the 'window.opener object'.
There are a few options when setting the correct 'Referrer Policy' and you need to be careful not to set a policy that is too strict if your site deals with affiliate links. The below 3 links are great resources to learn more about the different 'Referrer Policy' options:
Great article about Referrer Policy
More technical specs about Referrer Policy
More great info about noopener, noreferrer, and nofollow Values
Ultimately, it's up to you how restrictive you want your 'Referrer Policy' options to be. The below option is what I use for most of my sites:
Header set Referrer-Policy "no-referrer-when-downgrade"