Cross bracing, or bridging, is when you put two pieces of wood in an X in between your floor joists. It prevents the joists from twisting and stiffens the floor above. Most older homes have extensive cross bracing, as does ours. Modern home building legislation does not require it, unless the joists are a certain length and width.
A more common sight is solid blocking; this is when you just stick one piece of wood in there, perpendicular to the joints and connecting them in a sort of H. It's effective enough; cross bracing is sort of the ultimate solution, where solid blocking just does the job. When you've got wires in your ceiling, though, you need cross bracing. And, as most of the housebloggers can tell you, in a lot of old houses there are wires everywhere.
If you look at the photo above, you'll note that there is no subfloor in the floor above. That's actually the hardwood floor of the room above the dining room (which is, if you're curious, the temporary office, where I'm typing this right now). It's in great condition, with no warping or twisting, in no small part due to that cross bracing.