The Turing Test is named after Alan Turing. Alan proposed a test in the 1950's to distinguish a human from a machine.
The Turing Test used by Spam Sleuth is a simple test which is not easily automated by a computer. The test taker simply enters the letters shown into a box. The letters are partially obscured to make it more difficult for a computer to pass the test.
When a Turing Test is requested, an e-mail will be sent back to the sender, which requests that they click on a link and verify that they are human. Once they've passed this simple test, a message will be sent to your e-mail box which releases the original e-mail to your InBox.
The net effect is that you can set your spam screening even tighter (lower your spam threshold score) with assurance that if a good message is mistaken for spam, the original sender will get a chance to prove they are not a bulk spammer and their message will be delivered.