It may sound confusing, but it's actually quite simple: A new Gmail feature lets you attach an email or an entire thread to another email. 

In a blog post, Google explains that, in some situations, this could be more practical than forwarding several separate emails. "Sending emails as attachments allows you to write a summary email message to your recipients, and attach the set of supporting emails that recipients can directly open in their mail client," the post says. It's not a need I've encountered often, but it certainly doesn't hurt to have the option. 

To do it, you can start a new email and then just drag the subject line from a list of emails and the software will recognize it as a possible attachment (see video below).  Read more...