It uses "a cloud-based data service to transfer game data between mobile games and console games." You can visit the Nintendo Account website here to learn more (via Kotaku), but note that this is the Japanese version.
There is no word yet regarding when Nintendo Account registration will be available in other regions, like the United States and Europe.
In addition to Nintendo Account, Nintendo will launch a new rewards program called My Nintendo. No further details are available about this, but we're expecting to learn more in the coming months.
The first of Nintendo's smartphone games, Miitomo, will arrive in March 2016 through a partnership with DeNA. Previously, this game was expected to launch before the end of the year. On the console side, Nintendo plans to talk more about its mysterious NX console in 2016. It's rumored to be a console/mobile hybrid that uses "industry-leading" tech.