See the "Syntax for Function Definition" section on this documentation page. When you define a function the input arguments must be specified as variable names. Those name the variables in the body of the function into which the values the function's caller passes as input arguments will be stored. In that section note that the input arguments in the definition are one, two, and three not 'one', 'two', and 'three'.
'filename' is not the name of a variable. filename (no quotes) is the name of a variable.
When you call the function, that's when you can pass in a value like 'filename'.
