Meh, essentially it’s just writing “Telecommunicationsourcesurveillance” as a single word without the spaces to indicate it’s a singular thing being referred to (in this case the concept of directly listening on the source device before encryption happens). Might seem weird I guess, but you get used to it pretty quickly.

https://de.wikipedia.org/wiki/Telekommunikationsüberwachung#Quellen-Telekommunikationsüberwachung

It means “telecommunication source surveillance”.