Pyinex is a project to embed the Python interpreter in Excel. This is in contrast to the usual approach to linking the two systems, in which Python scripts run in an out-of-process COM server and Excel makes a cross-process call to use Python functionality. In Pyinex, Python runs from a DLL in the Excel process space.
This embedding offers the advantage of speed (no cross-process calls) and reduced fiddliness. There is no setting up of COM servers and their associated security - you simply write a Python script in a text file, and call it from within Excel.
This opens up the entire world of Python libraries to Excel users, and allows reduced dependence on VBA code, which is always hidden inside the Excel file. With program logic in separate, easily-viewable Python scripts, a new level of testing, visibility, and control of spreadsheets is possible. Excel can serve as a user scratchpad, UI, or graphing tool, and leave the "serious" computation to Python scripts.
These details are provided for information only. No information here is legal advice and should not be used as such.