A lighting bar, in a theatre, is a metal (normally steel or
aluminium) bar, usually the width of the stage, on which lights
are hung. Some lighting bars are just plain bars, and cables are
taped to them. Others have sockets built in, and with a
multicore, or multiple plug tails coming out of one end.  If the
bar is ready-wired, then it can be a lot more convenient, but is
also less flexible - you end up needing lots of short extension
cables if you need lights packed together more tightly than the
sockets are on the bar. On the other hand, taping your own
cables in can be more flexible, it is also a lot more work.