A Test Plug is used in the routine synchronization tests which Eva pilots undergo at Nerv HQ. The Test Plugs appear to be a row of three Entry Plugs set at an angle in a large pool of LCL, with the cockpit above the level of the LCL. While in the Test Plugs, the Eva pilots synch ratio is monitored. The Test Plugs are labeled "00" through "02" for the pilots of Eva-00 through Eva-02, respectively.
It is not clear just exactly how the pilots are capable of "synchronizing" if they aren't in their respective Evas or if the Evas are even involved directly. The Simulation bodies seen in Episode 11 seem to possibly be an extremely advanced analog of the Test plug. If that is the case, then like the simulation bodies, the Test Plugs may operate by actually connecting remotely to the actual Evangelion Units. This would give the benefit of being able to specifically monitor the pilots' synch ratios without having to go through the entire process of prepping the actual Evas.