QGIS now includes "buffers" for marker symbols. Just like label buffers, marker buffers are a halo effect around marker symbols designed to make the symbol more readable against different backgrounds.
Buffers are defined on a symbol level (not per symbol-layer), and are calculated and drawn using the shape of ALL the symbol layers in the marker. This makes them super-easy to configure, as getting the same result by hand would require many duplicate symbol layers and duplicate settings across these!
This change is designed to plug on of the (very few!) remaining large symbology gaps between ArcMap and QGIS, where ArcMap has had support for solid color marker "halos" for decades.