It is an argument of an automatic function injection

First of all, a normal function call looks like this:

That’s not handy to chain different function calls. We came up with infix function calls, that means we can move the map to the middle of the parameters and remove the parentheses. It looks like this now:

And the function map expects to receive a function in the right hand side. DataWeave automatically injects a function if you don’t write a literal one. The injected function will register arguments $ and $$ in this case, those corresponds to item = $ and index = $$.