A style ID is a map style's unique identifier.
When you create a style with Mapbox Studio, it generates a style URL that includes the style ID. A complete style URL, like
mapbox://styles/mapbox/light-v11, is comprised of three components:
mapbox://styles: points to the Mapbox Styles API
/mapbox: your Mapbox username
/streets-v12: the unique style ID
You need to know the style ID for the specific map style you are working with when you use the Mapbox Styles API.