The interface level sits between the
radiative level and the
convection level in the
interior structure of the Sun. As with most of the inner workings of the Sun very little can be observed, and as such most of the information available to scientists about it is sketchy at best.
This thin layer has become more interesting in recent years as more details have been discovered about it, such as the rapid chemical changes taking place, and how the shear flows that exist within the layer affect the solar magnetic fields. The interface level is also believed to have a influence on the solar dynamo effect that is found within the convection level.
Information taken from:
http://wwwssl.msfc.nasa.gov/ssl/pad/solar/interior.htm