PBX is the acronym for Public Branch Exchange. PBX is the technology that is used by a telephone provider to route calls. So if you are using multiple phone extensions (whether it is VoIP or not) PBX is the required technology.
In the old days PBX was provided as costly hardware that was stored onsite. These days, with the more efficient and secure cloud technologies and servers available remotely (on the Cloud), PBX no longer requires onsite hardware. Everything is now stored and managed remotely, on the Cloud, hence the term CloudPBX.