An automated testing harness
From the site:
The CTS is an automated testing harness that includes two major software components:
The CTS test harness runs on your desktop machine and manages test execution.
Individual test cases are executed on attached mobile devices or on an emulator. The test cases are written in Java as JUnit tests and packaged as Android .apk files to run on the actual device target.
The CTS includes the following types of test cases:
Unit tests test atomic units of code within the Android platform; e.g. a single class, such as java.util.HashMap.
Functional tests test a combination of APIs together in a higher-level use-case.
Reference application tests instrument a complete sample application to exercise a full set of APIs and Android runtime services