I'm pretty sure this is Apple's way of asking "Act as an IPv6 router and send router advertisements?". It's what you enable if you've configured a 6in4 tunnel endpoint on the Airport.
http://support.apple.com/kb/PH5098"If you chose Native from the IPv6 Mode pop-up menu, you can deselect “Enable IPv6 connection sharing” to not share the IPv6 Internet connection with devices on your network."
AIUI from a Google search, IPv6 Connection Sharing determines whether (when acting as a router, rather than a plain access point) the router gets just a /128 for itself, or whether it attempts to use DHCP-PD to get a /64 for its "LAN" side interfaces.
Presumably some providers are tight when it comes to v6 address allocation.
Sounds like unticking it just stops it acting as a v6 routerhttp://support.apple.com/kb/PH5098
NATs on IPv6 on iProduct.... That's good, NATs are good, I want NATs on IPv6 on my iPhone:http://www.youtube.com/watch?v=v26BAlfWBm8