Jitter refers to the variability in time delay of data packets over a network, which can negatively impact real-time communications like VoIP calls and online gaming. It occurs due to factors such as network congestion, improper queue configurations, and varying packet routes. Measuring jitter accurately is crucial for managing it effectively. Jitter mitigation techniques include using jitter buffers, upgrading network infrastructure, and implementing Quality of Service (QoS) settings.