Jitter is measured in milliseconds, but instead of measuring the time it takes your device to send data to a remote server and receive a response, jitter speed test describes latency differences between the flow of data to various client devices on your network. If jitter gets too high, it means that data isn't flowing to your device as efficiently as it probably should, and that can cause problems like buffering during streaming and video calls.
https://medium.com/@testworldonline/jitter-speed-test-58f1b4248f76?postPublishedType=initial