The Android Open Source Project (AOSP) is stored as collection of Git repositories, which are managed together using the Repo tool. Most Android development tools (such as GitHub) can't see the source code the way that it's laid out when it's checked out. Android Code Search is a tool that helps you view the Android source code as it's laid out when you actually use it.
Android Code Search makes it easy to navigate cross-references across all of AOSP by letting you to click through one part of the source code to another. This method of navigation helps you switch between Android's open source branches.
Android Code Search is similar to Google Code Search. For more details on using Google Code Search, see Getting started with Code Search.