Masking is a technique used to alter specific parts of your final output without affecting the rest of the scene. Unreal Engine's renderer sends many different render targets to the GPU which are put together to create the final image in real time.

In this Unreal Tips & Tricks video, we'll discuss the Custom Depth Buffer render target used in a Post Process Material, which allows us to quickly mask out any subject.

For additional help, please see the documentation: https://docs.unrealengine.com/Engine/Rendering/PostProcessEffects/PostProcessMaterials/index.html