The Android Open Source Project (AOSP) is a 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 on the system. Android Code Search (cs.android.com) is a tool that helps developers view the Android source code as they actually use it.
Code Search makes it easier to navigate cross-references across all of AOSP by allowing you to click through one part of the source code to another. This can help you switch between Android’s open source branches. Only the master branch has cross reference information in Java and CPP, but not Go.
For more details on using Code Search, see the Code search documentation on the Google Developers site.