Basically,
AV over IP is sending audio and video information over standard IP mediums.

AV over IP systems are now taking full advantage of the convergence of AV and
IT, so in a broader sense, it also now means providing the extension and
switching of video and audio sources over IP, such as the ability to distribute
4K/Full HD video, HD audio, and – importantly – control signals over a standard
Gigabit network.  The AV over IP solution
removes the limitations of distance and input/output numbers, offering greater
flexibility and scalability that are especially suitable for large-scale AV
installations. In addition, AV over IP solutions are a cost-effective means of
leveraging existing IT infrastructure without the need to use special or
dedicated cables.