Inline frames (iFrames) are an easy way to embed content from another site onto your own. They are also popular with website attackers because it allows them to easily load malicious content from their own servers. Attackers often use this feature to insert malicious content into compromised sites for the purpose of spam redirection, phishing, and distributing malware. These invisible iFrames are known to make page load times slower and can result in a negative user experience, but are otherwise harmless to users.”]
Source: https://blog.sucuri.net/2017/09/amazon-affiliate-cookie-stuffing-in-iframes.html