Jitter is variation in latency over a series of samples. Two connections can have the same average latency but feel very different if one is stable and the other swings sharply between fast and slow responses.
Real-time impact
Voice, video and online games prefer consistent delivery. High jitter can cause choppy audio, buffering or erratic game response.
Why it happens
Congestion, queueing, weak wireless links and changing cellular conditions can create uneven delay.
Improving stability
Reduce competing traffic, use wired networking where practical, improve Wi-Fi signal quality and avoid unnecessary VPN routing.