This is the name of the most recent virus or malware the Anti-Virus engine caught specifically in HTTP traffic. Since this is scoped to the web vector specifically, an admin uses it to know whether the most recent threat came from a drive-by download or malicious web content rather than email, which changes where follow-up effort should focus, for example browser hardening versus email filtering. For example, this field showing a trojan name known to spread via malicious ad networks would point the investigation toward web browsing behavior rather than email attachments.