It's the arity of the function. In Erlang there are no variadic functions, but functions with different arities can have the same name, so universal_server/0 takes no arguments, fib/1 takes 1 argument, fib/2 takes 2 arguments, the second arg may be the accumulator for a recursive Fibonacci, for example, and fib/1 may call fib/2 as an implementation detail.