Atrioventricular reentry tachycardia (AVRT) is caused by a macroreentry circuit involving the AV node and an accessory pathway. Normally, the AV node functions as the only true means of communication between the atria and the ventricles. In some people, however, a second (or multiple) tract develops elsewhere along the AV septum that allows another route of communication between the atria and the ventricles. This area is called an accessory pathway.
A good way to think about it is that an accessory pathway is like a backdoor into the ventricles. That backdoor does not have a guard on it like the AV node does, and so communication is unimpeded. In other words, the impulses move right through the pathway without any physiologic block or control of any kind. This lack of control can lead to some serious tachycardias, which could be life threatening in many cases. AVRT is just such a rhythm disturbance.
Accessory pathways have their own rates of impulse conduction and refractory periods. These variations will be important when we begin to consider how the macroreentry circuit is formed in AVRT.