Hi Fajid,
To answer the question fully, I will have to understand the context in which the 9:91 ratio was presented.
However, a 9:91 benefit cost ratio implies that for every 91 units of cost, there is only 9 units of benefit to be had, which means that justification for the project is going to be difficult.
There is often reference to cost benefit ratio's too, in which case you just interpret the values either side of the colon as cost to the left, and benefit to the right (other way round than in example above)
Hope this helps.
Regards