Field property? what is that? So in this video, he talks about making property some sort of "Field" for the new blender api
https://youtu.be/Mjy-zGG3Wk4?t=108

but what is that field? I dont find reference on any web page. seems like no one used that in the history of python?
Those are type declarations, and were introduced in Python 3.6. Blender 2.80 uses those to create the RNA properties. It was implemented by Campbell in 11 July 2018.