The Font Display setting controls what happens while a font is loading. There are four options, and swap is the default.
The browser will show invisible text for up to 3 seconds, it will then display the Google Font if it has downloaded.
The browser will initially show a system font, then once the Google Font has downloaded it will swap the fonts.
The browser will initially show a fallback font. If the Google Font is downloaded within 3 seconds it will swap the fonts. If the Google Font can’t download in 3 seconds or less it will continue using the fallback.
This mode is very similar to fallback, except the wait time is a lot lower. If a font hasn’t downloaded within 100ms the fallback will persist. To realistically load in 100ms a Google Font must already be in the visitors cache.