Error warning C275

Hi to everybody!
I am compiling the following code with the keil compiler version 8.18:

static portTASK_FUNCTION( prvIdleTask, pvParameters )
{ (void ) pvParameters;
....
}

and I get the following warning message:

warning C275: expression with possibly no effect

What means this sentence? The (void) is not a data type, why not the keil compiler give an error? only a warning?
Is it possible to solve this warning?

Thanks in advance.