Bitcode refers to an intermediate representation of a compiled program. The installation of iOS, tvOS, and watchOS apps by tailoring app delivery to the capabilities of the users particular device. Apple called this optimization,"app thinning". Note: Sliced apps are supported on devices running 9.0 and later; otherwise, the App Store delivers universal apps to customers. If you came across error like this:-

You must rebuild it with bitcode enabled (Xcode setting ENABLE_BITCODE), obtain an updated library from the vendor, or disable bitcode for this target. for architecture arm64 clang: error: linker command failed with exit code 1 (use -v to see invocation)

Solution :- Select your app "target". Type BITCODE in the search bar, it will display "Build Options". Under build options set "Enable Bitcode" to NO. Apple provided a complete documentation on their developer site. There they have explained about it with detailed steps. For more info visit here:- https://developer.apple.com/library/prerelease/ios/documentation/IDEs/Conceptual/AppDistributionGuide/AppThinning/AppThinning.html