下面這段英文來自於官方文件的如下位置：android-sdk-windows/docs/training/basics/firstapp/creating-project.html

app/build.gradle Android Studio uses Gradle to compile and build your app. There is a build.gradle file for each module of your project, as well as a build.gradle file for the entire project. Usually, you're only interested in the build.gradle file for the module, in this case the app or application module. This is where your app's build dependencies are set, including the defaultConfig settings:

• compiledSdkVersion is the platform version against which you will compile your app. By default, this is set to the latest version of Android available in your SDK. (It should be Android 4.1 or greater; if you don't have such a version available, you must install one using the SDK Manager
  .) You can still build your app to support older versions, but setting this to the latest version allows you to enable new features and optimize your app for a great user experience on the latest devices.
• applicationId is the fully qualified package name for your application that you specified during the New Project workflow.
• minSdkVersion is the Minimum SDK version you specified during the New Project workflow. This is the earliest version of the Android SDK that your app supports.
• targetSdkVersion indicates the highest version of Android with which you have tested your application. As new versions of Android become available, you should test your app on the new version and update this value to match the latest API level and thereby take advantage of new platform features. For more information, read Supporting Different Platform Versions.

         大概意思是：  Android 系統使用gradle 來編譯和構建你的應用。在你的專案的每一個module中都有一個build.gradle檔案，整個專案也有一個build.gradle檔案。通常情況下你只應該對module中的build.gradle感興趣，這裡指的就是我們的APP所在的module。你的應用程式如何構建將根據這個build.gradle中的設定來進行，一般build.gradle中預設包括下面的配置：    comipledSdkVersion : 用來編譯APP的android版本，通常設定為最新的版本。（你應該使用android4.1或者更新的版本，假如你沒有這些版本的話，你需要安裝並使用SDK manager來下載它們）。使用新的版本編譯APP時，仍然支援老版本的特性。把這個值設定成最新的版本就可以允許你使用新的特性，並且可以讓你的app在新版本的裝置上有非常好的使用者體驗。    applicationId： 專案的頂級包名。    minSdkVersion ： 指定的最小的SDK版本。就是我們的app所能支援的最老的版本。    targetSdkVersion：我們的產品可以執行的，經過測試過的，最高的SDK版本（比如說我們的APP最高支援Android 5.0）。隨著新的SDK版本的來到，你應該在新版本上測試你的app，然後更新這個值來迎合新的API,這樣就可以利用新版本的特性了。