You're seeing just the macro define_filter_adder, go back to Forage.QueryBuilder.Filter.AddFilterToQuery module for more information.
Link to this macro

define_filter_adder(filter_adder, n_max)


Defines a function that adds fields to a pre-existing Ecto query.

The function is named filter_adder (must be an atom) and will support up to n_max variables in the query.