Trying to find what is “SOP” I found the following “explanation” [www.sidefx.com]:

The SOP Geometry DOP pulls geometry from a SOP for use in a DOP simulation.

Can it be translated into plain English? Is there any glossary of custom words, invented by Houdini?

Here is a nice explanation:
https://www.sidefx.com/forum/topic/43515/?page=1#post-195103 [www.sidefx.com]

Regards
Bonsak

He knows what it means.

John Moncrief also did a nice clear breakdown at siggraph. Its at about 6mins in, but probably best to watch from the start:

https://vimeo.com/286242796 [vimeo.com]