Blending
Blending specifies to the GPU how the pixels that it will draw should be applied to the pixels that already have been drawn so far.
Blending is split up into the 'source' part and the 'destination' part. The source is the pixels that we are currently working with, the pixel we are rendering right now. The destination is the pixel that's stored in the framebuffer and we're going to be drawing into.
With blend modes, we can specify how we use and blend source and destination together.
This shader always uses the 'Add' blend mode operation. You may specify your own blend factors.