A "splat" mixes two or more textures using data from a third one (or elsewhere).
In this case, Splats 1-4 are the textures to be displayed/mixed, and the splatmap has the colours that determine which of the 4 are displayed (red shows splat 1, green splat2, etc).
As to why it doesn't work on any old mesh, I don't know.
IMHO unity shaderlab doccumentation is despicable.
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