It could be one of three things
1. Let's you use a third party controller or wheel (doubtful)
2. Let's you play 360 games on the XBox1 (wouldn't they use another word for that?)
3. Let's you take the system to Europe from America (also known as a transformer)
The only other converter I could think of is the one that converts your wireless controller in to wired one (also charges the batteries)
However, the description I am looking at on Amazon does not say anything about a converter, and what is included in the box is everything you need other than a game to use the thing. I do see how the Day One edition does say no converter, but the rest of the description only says special packaging as the only difference I see between the two editions.
http://www.amazon.com/gp/product/B00CMQTVMI/ref=XB1_console_HQP_image?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=hero-quick-promo&pf_rd_r=0X86QQCD343K3F0DAWWQ&pf_rd_t=201&pf_rd_p=1630167322&pf_rd_i=B00CMQTVMI#Xbox%20One%20Console
What's in the Box
- Xbox One Console
- Kinect Sensor for Xbox One
- Wireless Controller
- Chat Headset
- HDMI cable
- Power Supply
* Advanced TV hardware required. Games, add-ons and media content sold separately. Initial set-up and some games and features require broadband internet; ISP fees apply. Online multiplayer, Skype and some features also require Xbox Live Gold membership (sold separately) and/or additional requirements. See xbox.com/xboxone.