It doesn’t do anything.
Nintendo games usually don’t get a whole lot of updates- they launch feature complete and polished from the get go. The few updates they do release are usually substantial ones, adding some pretty major features and data to the games. However, this new update for Super Mario Maker is the exception.
It’s a 300MB patch for the game, which… well, it doesn’t seem to do much. It just seems to make some under the hood gameplay improvements and minor fixes. Whether it’s some glitch in the game that hadn’t yet been caught in public, or some kind of exploit that may have compromised the game, or worse, the Wii U system, that has been patched out, or just routine maintenance, is right now unknown.
Most likely, the update just lays the groundwork for future content, which is generally added to the game for free, like with Splatoon. Nintendo’s really got the post launch support part down for its games, that’s for sure.