Jitter refers to the variation in time between packets arriving, caused by network congestion, timing drift, or route changes. Consistent arrival times are necessary for high-quality real-time applications, like VoIP, making understanding jitter important for network professionals. High levels of jitter can result in packets arriving out of sequence, which negatively impacts the quality of the call. Knowing what jitter is and how to measure it is key for maintaining network performance, especially for services sensitive to timing like audio and video communication.