I have the definitions of computable (from my class notes) over here:

> 
> $f$ is computable if there is a computable function $\hat{f}:\Bbb N \times \Bbb N \to \Bbb Q$ such that, for all $n,r \in \Bbb N$
> $$
> \left |\hat{f}(n,r) - f(n) \right|\leq2^{-r}.
> $$
> 
> 
> 

I come from a non-math background, so could someone break it down for me? I understand what $\Bbb N$ and $\Bbb Q$ are. I am just mainly confused with the expression above.