Soft launching your game is the practice of releasing your new app in one or two select countries first. The aim is to get valuable feedback as to usage and purchasing behavior, identifying possible problems and estimating retention.
Obviously, the selected countries should be representative of your target audience, both in culture and affluence.
Also, your game should be complete, a soft launch is not a beta test.
It is vital to know exactly what you want to measure, then use analytics to track these events. The results will allow you to adjust and fine tune your game before its actual launch.
A/B Testing is a further tool implemented during a soft launch. It allows you to test different variants of your game, e.g. buttons, game speed or difficulty.
Many game review sites are keen to get an early look at a title in soft launch. This can be a great opener, getting the press interested. If they liked it, they will most likely review your game when it finally released.