What does *provably good* mean ? Does it mean not good for practical scenario and only good theoretically ?

> 
> None of the hash functions that we would be discussing in today's
>  lecture are provably good hash functions .
> 
> 
> 

I see it [here](http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-046j-introduction-to-algorithms-sma-5503-fall-2005/video-lectures/lecture-7-hashing-hash-functions/) at 39:06 time line.