The ZWO T2 to T2 adapter is a short adapter with a male M42 thread on either side.
When screwed into the front of your ZWO ASI astronomy camera it changes the gender from a female M42 to a male M42 thread. For example, this will allow you to attach your ASI camera to a filter wheel.

The T2 to T2 adapter allows you to couple any 2 devices that both have a female M42 (T2) Thread.