The Blending section defines how the shader should blend on top of pixels that are already drawn. These options are set by the Render Preset selected at the top of the shader, and almost always need not be set manually.
These blend options are exposed separately for the base pass and the add pass, as the add pass needs to blend on top of the base pass.
RGB/Alpha Blend Op
What blend op to use for the RGB and A channels. This is almost always set to
Add. See the Unity documentation for more information.
Which blend factors to use for the source and destination. See the Unity documentation and Render Presets page for more information.