A font stack is an ordered list of fonts containing a primary font and optional fallback font(s), as defined within a layer. Within a font stack is a combination of primary and fallback fonts that 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 stack, then it will try to use the next one until it can find a font with that character.
A style object itself does not have a main font stack. Rather, individual layers have their own font stacks that are defined within the
text-font property of a symbol layer, as detailed in the Mapbox Style Specification.
You can manage the font stacks used within a style to improve rendering time and reduce the size of an offline map.