Gmail users are getting a new feature that allows them to add a new kind of attachment to their emails: other emails. This can be seen as an expansion of the typical method of forwarding emails to other users, and indeed, the option will show up as "Forward email as an attachment".
The biggest benefit of this capability is that it makes it easier to forward multiple emails at the same time, so if you want to compile a list of messages around a specific topic, you can select all the emails you want to include and send them all in one message. The feature also supports drag-and-drop from the user's message list.
Once emails are attached to a message, they're shown as .eml files, and there's no limit to how many messages can be attached. Clicking these files will open them in a new window, so you can always go back to the initial message.
This new feature will be available to all G Suite editions, and it requires no actions from administrators. The rollout starts today for Rapid Release domains, and it'll take up to 15 days for it show up for those users. For Scheduled Release domains, the feature will start rolling out on January 6, so you'll have to wait a little longer.