A font stack is an ordered list of fonts that contains a primary font and an optional fallback font or fonts. In this example,
"Open Sans Regular" is the primary font and
"Arial Unicode MS Regular" is the fallback font:
["Open Sans Regular","Arial Unicode MS Regular"]
The font stack is defined within a map's symbol layer. The font stack defines the order in which the renderer should try to find the characters used within the layer. If the renderer cannot find a character within the first font, then it will try to use the next one. It will repeat this process until it can find a font that has that character.
A style object itself does not have a main font stack. Rather, individual layers within the style object have their own font stacks that are defined within the
text-font property of a symbol layer, as detailed in the Mapbox Style Specification.