It means if you want to replicate how ShadowMap lighting previously looked, you set LightingStyle=Soft and PrioritizeLightingQuality=true. Currently there seems to be an issue where the default LightingStyle is Realistic which is what seems to be happening in your place.
“Unified” lighting means that the ability to select a specific lighting technology is being replaced with these two new properties. Certain lighting settings are being decoupled from technology, giving you more control over lighting and making it easier for Roblox to add additional features and performance enhancements.