The Return/Risk ratio gives an estimate of how much Return is obtained per ''unit of risk''.
It enables you to compare strategies in regards to the risk they can handle. It is calculated by carrying out random strategy simulations which trade the same assets as the average strategy, and comparing them.
To have an idea of how good a strategy is:
Above 1, the strategy beats 68% of random strategies, it is good
Above 1.65, the strategy beats 95%, it is very good
Above 2, the strategy beats 99%, it is impressive