What are VoIP protocols?

What specifically don’t you understand about them?

VoIP does not work with a single protocols. It uses a large set of protocols which is collectively known as VoIP. Understanding all of them is not possible in one sitting. You have to spend a little time on internet and a tutor as well.
Source:
http://www.guidingtech.com/mobiles/understanding-voice-over-internet-protocol-voip-and-how-it-works/

As you know VoIP is the abbreviation of voice over internet protocol. All the protocols that are using to run internet can be used in VoIP. However, providers like Axvoice, Vonage and Packet 8 use different protocols to develop their calling solutions. The use of protocols depends on the requirement of their clients also. That means, they might use or leave some protocols according to the requirement.