A.Enum.max_by

You're seeing just the function max_by, go back to A.Enum module for more information.
Link to this function

max_by(enumerable, fun, sorter \\ &>=/2, empty_fallback \\ fn -> raise(Enum.EmptyError) end)

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Specs

max_by(
  t(val),
  (val -> key),
  (key, key -> boolean()) | module(),
  (() -> empty_result)
) :: val | empty_result
when val: value(), key: term(), empty_result: any()

Returns the maximal element in the enumerable as calculated by the given fun.

Mirrors Enum.max_by/4 with higher performance for Aja structures.