The Apple Hardware Test, or AHT, is a hardware test on Macs that tests the internal components of a Mac, such as the logic board and RAM. It does not perform any tests on external devices connected to the Mac, or software installed on the Mac.
The icon shown whilst AHT is testing the Mac.
You can read more about Apple Hardware Test in Apple KB article HT1509.
AHT was replaced by Apple Diagnostics in Macs released after June 2013.