google 文档说明 对应地址: https://developer.android.com/studio/build
1、buildscript是gradle脚本执行需要的依赖
2、allprojects里是项目本身需要的依赖。表示项目里面每个build.gradle 里面的库都会依赖用这些仓库。

顶层构建文件 :顶层 build.gradle 文件位于项目的根目录下，用于定义适用于项目中所有模块的构建配置。默认情况下，顶层构建文件使用 buildscript 代码块定义项目中所有模块共用的 Gradle 代码库和依赖项。以下代码示例说明了创建新项目后可在顶层 build.gradle 文件中找到的默认设置和 DSL 元素。

/**
 * The buildscript block is where you configure the repositories and
 * dependencies for Gradle itself—meaning, you should not include dependencies
 * for your modules here. For example, this block includes the Android plugin for
 * Gradle as a dependency because it provides the additional instructions Gradle
 * needs to build Android app modules.

翻译软件后解释：
buildscript块是对Gradle 配置存储库和依赖。其意味着，对你的模块 module 不应该包括依赖项。例如，这个块包括Gradle依赖项的Android插件，因为它提供Gradle构建Android应用程序模块所需要的额外的指令。
 */

buildscript {

    /**
     * The repositories block configures the repositories Gradle uses to
     * search or download the dependencies. Gradle pre-configures support for remote
     * repositories such as JCenter, Maven Central, and Ivy. You can also use local
     * repositories or define your own remote repositories. The code below defines
     * JCenter as the repository Gradle should use to look for its dependencies.
     *
     * New projects created using Android Studio 3.0 and higher also include
     * Google's Maven repository.

  
翻译软件后解释：
repositories块用于配置Gradle搜索或下载依赖项。Gradle预配置支持对远程JCenter、Maven Central和Ivy等存储库。您也可以使用本地存储库或定义您自己的远程存储库。下面的代码定义JCenter作为Gradle用来查找其依赖关系的存储库。
     */

    repositories {
        google()
        jcenter()
    }

    /**
     * The dependencies block configures the dependencies Gradle needs to use
     * to build your project. The following line adds Android plugin for Gradle
     * version 4.0.0 as a classpath dependency.

翻译软件后解释：
dependencies块配置Gradle建立项目所需要使用的依赖项。下面一行为Gradle添加了Android4.0.0插件版本作为类路径依赖项。
     */

    dependencies {
        classpath 'com.android.tools.build:gradle:4.0.0'
    }
}

/**
 * The allprojects block is where you configure the repositories and
 * dependencies used by all modules in your project, such as third-party plugins
 * or libraries. However, you should configure module-specific dependencies in
 * each module-level build.gradle file. For new projects, Android Studio
 * includes JCenter and Google's Maven repository by default, but it does not
 * configure any dependencies (unless you select a template that requires some).

翻译软件后解释：
allprojects块是配置项目中所有模块使用的依赖项和存储库，例如第三方插件或者库。但是，您应该在每个模块级别build.gradle 文件中配置特定于模块的依赖项。对于新项目，Android Studio默认情况下包括JCenter和Google的Maven存储库，但它没有配置任何依赖项（除非选择的模板需要某些依赖项）。
 */

allprojects {
   repositories {
       google()
       jcenter()
   }
}