Question: How to implement mod?

I need to find factors of the eq, but unable to understand use of mod.
> `mod`(Factor(x^5-1), 2);

output redirected...

> `mod`(Factor(x^5-1), 7);

powers= 4 3 2
(x + x + x + x + 1) (x + 6)

> `mod`(Factor(x^5-1), 5);

powers= 5
(x + 4)
 
which value of mod should I use?
How do I get to know that?
Thanks in advance.
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