Soft launch of a mobile game is where game publishers test their apps in selected countries. It is a very common technique among game publishers before hard launch in order to have a chance to:

• Acquire first users for a much less cost than hard launch 
• Collect feedback from real users from selected countries
• Find out and fix in-app bugs, performance issues, problems in user experience 
• Test metadata (app icon, app screenshots, app preview video, etc.) and ad elements (ad creatives, CTA, etc.) to see and understand and interpret conversion. 
• Understand the main metrics before global launch including CPI, Retention Rate, and more. 
• Collect insights and cues about the global launch marketing strategy (deciding on the countries, optimizing mobile ad elements and bids, etc)
• Test app monetization strategy to make improvements and optimizations
• Decide if the game should be killed or hard launched

For a successful soft launch campaign:

• Time period for the campaign should be pre-determined.
• Metrics mobile marketers and app developers want to test should be tracked carefully.
• Country and the game category need to be determined.
• Target audience should be worked closely. 
• Ad creatives and CTAs should be created and tested frequently.