Blending

Blending refers to how a shader blends with the background.

Common blending techniques are additive, translucent and multiplicative. It performs various operations on the pixel using the Source color (the thing you are working on) with the Destination color (the pixel behind what you are working on).

You can read about some examples and descriptions of how the Source and Destination are handled in different modes here: https://docs.unrealengine.com/en-US/Engine/Rendering/Materials/MaterialProperties/BlendModes