The Session Initiation Protocol (SIP) is an IETF-defined signaling protocol, widely used for controlling multimedia communication sessions such as voice and video calls over Internet Protocol (IP). That the definition of SIP according to Wikipedia. This definition is only the beginning, as SIP is a very complex protocol.
You can check IETF RFC, Wikipedia, Cisco.com for details, but most probably you’ll not remember too much about SIP, except if you are dealing with this protocol daily in your work field. As a network engineer you should know at least what is SIP and what it does, so here is a short video that explain SIP fundamentals: