How to generated random BigInteger numbers between 1 and anything?

Im trying to generate random numbers and i using BigInteger class to check if the generated numer is prime with probablePrime();

Here is my code:

btnRun.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Random prng = new Random();

            Toast.makeText(getBaseContext()," " + BigInteger.probablePrime(10,prng), Toast.LENGTH_SHORT).show();

        }
    });

I dont really understand the first parameter in the probablePrime() wich in my case is 10. I Know its the byte off the int value, is this effecting the range of the generated numbers? Im only get values from 523-1003 BUT i want to generate random numbers from 1-anything and check them in the probablePrime(); method.

How is that possible?