What are $\displaystyle \mathcal{AB}^{op}$ and $\displaystyle \mathcal{GRP}^{op}$ naturally equivalent to?

Okay I'm stuck in this one, I know what a natural equivalence is, but my problem is I don't really understand what $\displaystyle (.)^{op}$ does, I know it reverses all arrows in the Category but I don't get what changes by doing that.