Toni,
The "leader" is the bus/train which is supposed to precede the one you
are one.
Times you're likely to hear about a vehicle's leader on Muni:
* "Where the hell is my leader?"
Your leader is AWOL, and your bus will be jammed with angry riders who
have had to wait for ages, and your bus will run slower because it
takes increasingly long to load and unload the increased numbers of
surly passengers, making the huge gap between you and your "leader's
leader" even worse. It seems this is almost the normal operation mode
these days.
* "Bunching"
You catch up to the leader, because of breakdown, congestion, poor
driving, unscheduled breaks, or overloading in the leading bus which
causes it to fall behind schedule. Since your bus will _typically_
run faster as what should be its passengers are being swept up by
another vehicle just ahead of you, the following bus will typically
run faster as the leader continues to slow, a feedback which leads to
the bunching with which we're all so familiar.
* You see your leader coming the other way on the route, and you know
you're still 15 minutes from the turn-around. So now you know that
there's at least a 1/2 hour gap in service, and either the operator of
the lead vehicle is ahead of schedule (which should be a big no-no,
except that Muni has decreed that passengers aren't allowed to know
its secret schedules anyway) or your vehicle is way behind schedule
(and you're likely to run more slowly because of extra passenger load,
and you're more likely to have your following vehicle catch up to you.
See "bunching" above.)
You should feel free to ask Muni drivers about this sort of thing;
most are helpful, many appreciate any sort of conversation, and some
are interested in their job and in doing their job well and would like
more people to know about how it's done and what obstacles they face
in doing it.