about "assignment to free variable"

[Michelangelo Rodriguez]

Hi All,
When i compile a piece of code the byte-compiler reports a warning
telling me that i'm trying to assign to a free variable:
But the variable in question is, atleast i think, referred to a local
variable because it is an argument of the function.
So, what it means exactly?
The code works fine even if compiled.
If it is necessary i can post the code, even if it is large.
Thanks for your replies and best regards.
Michelangelo