Jitter is one of three key video conferencing performance metrics. The other two are packet loss and latency.
Jitter refers to unwanted variation when packets are received. It is the percentage of variation in the video transmission rate. data can be buffered accordingly if there is a traffic delay, however, processors can get overloaded when the delay continues to change and this drivies up latency and packet loss. This can result in "blockiness" of the video as well as a frozen or jerky appearance. A good rule of thumb for jitter is < 20 msec for a high-quality video conference experience.