From SOuP wiki
It is a blend of multiple things. To some point it is like scriptNode but on steroids with native python support. The AE Template provides extensive management over the input and output connections, there is also a text editor right there. It can access mesh and texture data connected to the input. Provides wrapper classes for vectors and matrices that make it very easy to work with that stuff. Also wrappers are provided for mesh and texture data. It is somehow similar to the SOuP point node, but applied to transforms. You can control lots of transforms easily and make them react to many scene events or other objects. Some people asked in the past about Cinema4D moGraph like functionality that can be used with transforms in Maya. This node can do stuff like that.
Check the "notes" section in the AE.
Source the AEpyExpressionTemplate.mel script or copy/paste it in the script editor or put it in directory that Maya scans for scripts. Then load the pyExpression.py file as Maya plug-in.
Credit goes to David Corral for the idea + implementation of the main framework. Peter participated with code for mesh data handling, texture sampling and the AE Template mechanics.