I know this is not absolute value, so what do the parentheses mean? If it was absolute value, it would have lines so I can rule that out.
And I guess I should mention, the question I was asked is "Enter the opposite of this number in the box." So, that's the context.
$-(-6)$
This means negative, negative $6$. As you probably know negative times negative is positive, so this is simply the value $6$ (positive six). The reason for the parenthesis is that the convention is that we do not have multiple operation signs in a row, e.g. signs like "$+$","$-$", and "$\times$". In a way you can say that every two negatives "cancel out", so
$$\require{cancel}-(-6) = \cancel{-}(\cancel{-}6) = \fbox{6}.$$
Now to answer the added question on what the "opposite number" is, I'm assuming that it is meant "the negative of the number." The question then is "What is the negative of $-(-6)$"? Since we have already resolved that $-(-6) = 6$, this question is the same as "What is the negative of $6$?", which is simply $-6$.