Jitter is a term used to mean the variation in lag over time.
In very simple terms, if you were to ping a network host 2 times and each time it responded in 100 milliseconds, we could say that its jitter is 0 ms. If instead, the 2 ping times were 75 and 100 milliseconds, we could say the jitter is 25 ms. Of course the jitter calculations performed in BZFlag are a little more complex than that, incorporating averaging and smoothing over time.
Coders interested in jitter in BZFlag's context may wish to study LagInfo.cxx from the BZ source. It can be found in bzflag/game/Source Files/ (VS08) or src/game/ in a ordinary file browser.