TF is a test harness used to drive Android automated testing. It runs on test hosts and monitors the connected devices, handling test scheduling & execution and device management.
Other test harnesses like Compatibility Test Suite (CTS) and Vendor Test Suite (VTS) use TF as a basis and extend it for their particular needs.
More information at: https://source.android.com/devices/tech/test_infra/tradefed/
See more details about Tradefed Architecture at: https://source.android.com/devices/tech/test_infra/tradefed/architecture