A layer in Mapbox GL JS is a visual element that displays data from a source. A layer is defined by its type, such as circle, line, fill or symbol, and contains a reference to the source it displays. A symbol layer displays text, icons, or both. Layers that reference vector tile sources contain a reference to the source layer within the vector tile style. They can also contain filters, which are Mapbox GL expressions that determine which individual features get displayed. Other layer types include raster, fill-extrusion, heatmap and hillshade.