Jitter is a term for the variation in lag over time.
In very simple terms, if one were to ping a network host 2 times and each time it responded in 100 milliseconds, one can say that its jitter is 0 ms. If instead, the 2 ping times were 75 and 100 milliseconds, one 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.