What is the __ doing in this snipped? "*{roles[__${roleStat.index}__]}"

This doc says (on page 26):

Preprocessed expressions are exactly like normal ones, but appear surrounded by a double underscore symbol (like __${expression}__ ).

So it's for preprocessing an expression before it's further used in whatever it's surrounded by.

EDIT: Let's additionally clarify what this does in your scenario (if it wasn't clear, yet). It's processing ${roleStat.index} to get a literal index number, to then use it as the arrays index.