SIP ( Session Initiation Protocol ) is a protocol used for initiating/maintaining/terminating real-time sessions related to voice, video and messaging apps.
You need to understand SIP if you have any interest in information security, because it’s a signaling communications protocol. It’s often used for managing multimedia communications like voice and video calls over Internet Protocol (IP) networks such as VoIP. SIP is fairly secure, but it can be compromised using Wireshark and one or two other tools.