I don't know if it's simpler terms, but the actual definition of a robust type is in the "what is unsafe" section (which can be found from the glossary). I highly suggest reading "what are types" first unless you have strong type theory background.
In a few words if you understand the quick start enough: unsafe types have values that the type system doesn't see, and robust types are missing values that the type system believe can be present.
This is described here and initially here. There's also a mention here. Hope this helps.