A parent material in Unreal Engine is a master setup you build once, where you plug in common maps like Base Color, Normal, and Roughness and expose any values you want to adjust later. From this, you create material instances that let you change things like color tints, roughness levels, or texture maps without recompiling the shader. You can also migrate or export your parent materials and their instances to other projects, which lets you reuse your material setups and keep a consistent look across multiple scenes or games.
https://dev.epicgames.com/community/learning/tutorials/VE7D/unreal-engine-parent-material-and-instancing