I can explain it! Or I can try atleast.
NXT is perhaps the first second gen crypto coin rewritten fully from scratch on Java and JS. It has a decentralized asset exchange ontop of it where people can create assets or essentially other coins, without having to worry about security. Divident payouts, time controlled smart contracts among other things are enabled by it now.
The multigateway (MGW) is a way for people to trade altcoins for assets that mirror the altcoins. Centralized exchanges use this same system. They create a mirror asset of say, Darkcoin, and let you trade it on their website. They hold your 'real' DRK on their accounts.
MGW works the same way, except it is cheaper, easier to log into and use than most and it's storage is decentralized. It uses 3 servers and 2/3 multisig, and these servers must agree for withdrawal to occur. I'm not certain of the details beyond this, or not can I really verify code, but, Blackcoin, litecoin, dogecoin among others are already using this service and I expect more to come!
Check out this article on the Supernet that just came out, might help clarify things.
http://bitcoinmagazine.com/18167/what-is-the-supernet-jl777s-vision/