It measures the risk:reward ratio, showing you the average reward you are getting on each trade in the selected currency. So if you have a risk ratio of 1.5 in EURUSD, it means that for every 1 unit of the trade you are initiating, you're expected to get back 1.5 units.
More over, you can see from this statistic which currencies are your more profitable than others and based on that you can decide in which currencies to increase your position sizing, and which to lower, which will eventually increase your profitability.
See Also
Is MyFxBook safe to use?