Font stack tells the browser which font to use in order of priority. If the first one is not available in the system and not provided by the site, browser goes for the second one, etc.
Nunito and Lato are Google Fonts - so they can be loaded from google as webfonts and will work in any browser and OS, but will request a font from Google every time. So Google fonts increase site loading time.
Also some countries like China block google and won’t load the fonts. So their browsers will drop to Arial or even default system sans-serif type.