The Nintendo Wii sold really well when it launched back in late 2006, so when the Nintendo Switch has two-day numbers better than the Wii, that is a very good sign for Nintendo's new console hybrid.
While there are no official numbers yet, New York Times writer Nick Wingfield said in a series of tweets that Nintendo's Reggie Fils-Aime confirmed that the first two days of sales for the Switch outpaced the Wii two-day launch totals. Given that Switch's launch was not during the holiday season makes the revelation even more impressive.
As for The Legend of Zelda: Breath of the Wild. Fils-Aime (via Wingfield) said that the game was also the top-selling standalone game in Nintendo history. Standalone means a game that isn't specifically bundled with the console like Wii Sports. The game that it beat was Super Mario for the Nintendo 64.
A pretty good start for the Switch and The Legend of Zelda: Breath of the Wild. Official numbers should give us an even better picture when Nintendo offers something concrete.